I get mine here, Canadian $ are cheaper than US, but still a bit pricey.
Batteries & Accessories : TBOLTRC.COM, - Giant Scale R/C! Hobby King has smaller ones like 800 mah square packs for cheaper. They are not as bulletproof as the A123 type. You need a special charger for them, as they are 3.3V per cell, but not as explosive as the LiPo. They are LiFe, Lithium Ferrite. I still make mine up from dollar store packs, but now we have $4 stores, so the last ones I got were the newer style NiMh ones that have a long shelf life. You don't have to charge them within a day of flying. Enelopes are a common brand, but I got something similar much cheaper here for about $1 a piece on sale. 1200mah AA or around 800mah AAA size.