That's really up to you, there are a number of good
batteries. AW and Redilast are among the best from those that test them. AW is available at lighthound.com and Redilast at redilast.com of course you'll pay more for these so it's up to you to figure out if it's worth it or not.
Ultrafire and Trustfire are probabaly the most popular because of the price point. For those check madvapes.com as well as many other places, they are easy to find.
Edit...check the battery you're looking at to see if it's flat top or button top. Some mods won't work with a flat top, some will, you'll just have to see on the specific mod. If you stick with button top and protected you'll be ok.