There's not a real good reason to go past 12ga, and even that may be more than you really need (particularly for shorter runs). I can't recall right now if my surrounds are run with 14ga or 16ga. If you are looking for a source online, I just ordered some short pieces from Blue Jeans Cables with bananas already on them (found that one of the runs from amp to wall plate for my surrounds had a loose connection on it and decided to replace it with something nicer). They also sell unterminated wire, as does MonoPrice.