I'm currently using the 4Sevens batteries in mine, although I have some Titanium Innovations double sticks on the way to try out. Both of these are manufactured in China. Reports from the batteries forum say there's only one manufacturer in the US, which means the USA-made batteries should be pretty consistent regardless of brand. You can get a good deal on the Battery Station ones via the CPF Specials thread.
Poke around the batteries forum and you'll get more recommendations and safety warnings, along with reports of testing and failures.