after my bad experience with third party battery i buy mine from Nintendo.com
Lites
http://www.nintendo.com/accessoriesds#ds-lite-accessories
Phatties
http://www.nintendo.com/accessoriesds#ds-accessories
their 15 bucks + shipping kinda on the high side but i personally think OEM battery is great
oh i did buy about 4 third party 1600mah battery to test out, all 4 were worthless
3 lasted 1/4 the original ds battery time if not less
1 was a complete dud, didnt accept charges
(ones i tried werent dragon brand so dont know about those)
best thing i have to show from it is 4 mini screw drivers rolling around my table somewhere, now their great for ds and other things
*Edit*
Opps not online... hmm try gamestop or local videogame store, or buy it in asia
btw Nintendo ships 1-2 day after order via UPS, also they usually ship from washington state for me so its pretty fast, they also offer faster shipping method for more $