Google "magicshine battery" - there are a lot. MagicShine is still in business and their packs are compatible. DX sells them, and probably others as well. There are MANY for sale on ebay including from US sellers, and including greatly improved versions.

If, as your handle suggests, you're not averse to doing a little DIY, ANY 7.2 volt LiIon pack with protection circuitry built in can have the proper connector pigtail soldered on. You can hack one out of the easily-available MS extension and Y connectors.