The pack is manufactured by a Japanese company called Thanko and is due for release there soon. Currently there no news on a release anywhere else.
Thanko's lithium-ion battery and USB charge cradle, which can draw power from one of the Wii's USB ports, are retailing in Japan at ¥3,480 (US$29) per set or at the slightly discounted rate of ¥6,480 (US$54) for two sets. A three-and-a-half hour charge will, according to Thanko's press release, keep a Wii Remote powered for 25 hours.
Most likely we will be seeing a series of these chargers pop up over the next few months. No one likes having to replace batteries, what were you thinking Nintendo?