I've used these two:
Smallest size, TP 45C 850 mah, but also shortest run time. This is just to show you the battery, I'm sure better deals can be had elsewhere:
http://www.helidirect.com/thunder-p...1v-45c-for-trex250-wjst-connector-p-16390.hdx
Hyperion 35C 1100 mah. A bit larger, but still a great battery. Again, this is just to show the battery, better deals can be found elsewhere:
http://www.allerc.com/product_info.php?products_id=4535
These are more near the top of the price range, but both are high quality. There are many batteries priced much lower.
There are many batteries which will fit the plate, usually in the 1100 to 1500 mah range, wether you have room or not will depend on wheel offset, how much steering you have, etc. Look around for some build threads that show these batteries and others mounted, also look in the electronics section, there is a lot of info there.