Where can you get them and is it free?

Oh, and how do you put them on an SD card?

Do you have iTunes? If you do, what you do is that you put your normal music files (I'm guessing MP3's?) on iTunes and right click on them. It'll give you the option to convert the files. It'll then take about 30 seconds or so to convert the file and will create a new one that's in AAC format. Just copy the converted version of the song to the SD card (Use a device which is SD card compatible like a Digital Camera if you don't have a built-in slot on the computer) and it should work.

@Toon Link: you'll need iTunes for this.

1: Open iTunes. Choose Edit, then Preferences from the drop-down menu.
2: Click the 'General' tab, then the 'Import Settings' button.
3: From the drop-down menu there, choose the format you'd like your songs converted to (AAC), and i believe there's also a drop-down for bitrate settings and whatnot. Once you've got everything to your liking, click OK to save your settings.
4: In your iTunes library, highlight the song(s) you want to convert. Right-click it/them, and choose 'Create AAC version' or whatever it says. Then sit back and relax and wait for your file(s) to convert.
5: When your files are all converted, you can find them in your iTunes folder (if you can't find them, just search your computer for '*.m4a'; that's the file extension that iTunes saves AAC-format files to on my computer iirc).

As for putting your newly-converted files onto an SD card, you'll need an SD card reader (some newer computers/laptops come with SD card readers built in, but other computers will require an external SD card reader that will likely plug in to your computer via USB)...
1: If necessary, plug in your SD card reader... if it's already built-in or hooked up, just insert your SD card into the reader slot.
2: A new volume should pop up after a few seconds under My Computer or whatever, and it will likely identify itself as an SD card. Open it.
3: Locate your AAC-converted files on your hard drive, and open that window.
4: Drag and drop/copy and paste the music you want on your DSi from your hard drive to the SD card.
5: Organize your music into folders if you like. The DSi will read up to several folders deep, though it will display only one folder deep in the DSi Sound menu (all the deeper folders will be brought up into the one same rotating wheel of available folders, so take care not to use the same name for multiple folders on the SD card, even if they've got different names for their parent folders).
6: When you're done mucking around with the SD card, dismount the volume and eject it from the reader. Pop it into your DSi and have fun with your music. :3

When I tried putting AAC files onto my SD card, it wouldn't let me. Are you able to do it via a camera, or do you need an adapter?

@edhe: check your camera's docs... they may say something about files being transferred to the camera SD card via USB have to be a specific range of formats or something. either way, if it's not letting you do it, then you probably do need a separate reader. don't worry, they're pretty cheap. :3

What the hell is wrong with mp3 huh?! Why?! _{stoopid apple}

Read this and find out. this here is also good reading on the subject. :3

@theblackdragon: I knew about DRM and the increase in quality, but didn't know about the sticky issues surrounding the mp3 patent.

I have nothing against the AAC format as such, more the company who has standardised it. I really dislike the bloatware that is iTunes, and the whole iPod revolution _{(other mp3 players are available)}, thank goodness for Media Monkey and the like.

The iPod is simply brilliant design as is iTunes, but I guess you cannot make everyone happy...

The iPod is very user friendly yes, which is why I dislike it. As for iTunes, I can only assume you have it installed on a Mac, which it is meant to be on.
I am happy, just so long as there are no Apple products in my vicinity.

Jealousy is such a sad thing...

But seriously, iTunes on Windows suck. The fact that AAC files are fairly useless outside of iPods and a select few music players is also annoying. But still, at least they didn't choose OGG.

Jealousy you say Sean? I think you have misunderstood me somewhat. I'm trying to be polite about my feelings toward Apple, please don't make it more difficult than it already is. Yes, a lot of people think the whole iPod/Apple scene is super cool, yet others, like myself, would rather burn in hell than own anything made by that damn company. Yuk!

You envy the ability of others to appreciate good design. But by all means, surround yourself with black squares of plastic festooned with buttons if that's what you like...

I think there's some confusion here. I'll try to clear things up.

• AAC is the legitimate successor to MP3 and is supported by a wide variety of software and portable players. If you've ever heard of an "MP4 Player", that's what they're talking about. The file is an m4a (MP4 Audio) but the actual data is AAC.
• If you have recently purchased music from the iTunes store, it can be dragged directly from iTunes onto a Windows Explorer window. This will copy non-DRM AAC music files that can be played by the DSi.
• If you purchased your music back when Apple DRMed their files, you'll need to upgrade your songs before you can play them on your DSi. I don't remember the exact link in the iTunes store, but it's somewhere on the right-hand panel. There is a small fee to upgrade your files.
• If you have music purchased for a PlaysForSure device, you're SOL. Sorry, Microsoft screwed everyone over with that one.
• If you have MP3 files, follow BlackDragon's instructions to convert them. The one change I'd make is that you can drag the files directly from iTunes to the SD Card window.
• The DSi will ignore m4p (DRMed iTunes files) if you accidentally put them on the SDCard. So feel free to drag and drop everything.
• If you don't want to use iTunes, there are a number of shareware programs that will convert MP3s to AAC/MP4 files. Just be aware that raw AAC files (the ones with the .aac extension) won't work right on the DSi. Make sure you choose a program that outputs the correct format. If you get it wrong, the DSi will display the file but won't be able to play it.

Hope that helps.

This amuses me.

I agree with Sean.

Here's how I do it:
1. Put the MP3 file onto iTunes.
2. Right click and hit, "convert into ACC file" or something like that.
3. Go to the folder, "My Music"
4. Go to iTunes music
5. Right click and chose the drive where the Sd card is.

Looks like this is a problem for DSi owners who play music on their DSi. I feel for you. I use an iPod Nano 3G (the little square one) and it plays both MP3 and AAC just fine -- there's a solution in there somewhere...

One thing Apple products are not, is well designed (though the gradual drip-feeding of features/improvements is good financial design), they may look nice and shiny, but there is not much going on inside. Each to their own of course, but I prefer I features over design, I don't want ease of use like the masses. My old Walkman phone does everything an iPod and iPhone does combined, it even does applications (I'm gonna punch the next iPhone user who shows me their latest 'app'). And don't get me started on the Mac/PC thing, as I am worn out there, as are many IT professionals I know, who dislike Apple even more than I.

This could go on forever, as it has in many other forums on many other sites. Blah Blah Blah de Blah.

If "IT Professional" means "Windows Admin" then I question the use of the term. UNIX admins, like myself, clearly prefer the one with the more solid OS under the hood which is OS X. I guarantee there's a lot happening under the shiny shiny exterior.

I'm in the chatroom if you feel the need to be harangued some more (oh and the Sony Ericsson must have the worst phone interface I've ever seen -- Nokia or even Sagem for me over that)!