Well,
for personal use,
I use Google image search with search phrases like
"cool avatar" "manga avatar" "comic 80x80" "32x32 avatar" and so on...
include a phrase for the topic you want plus the word avatar or the needed image size in pixels.
for example, if you want an avatar of a scuence-fiction theme, try something like
"scifi 80x80" or "sci-di avatar" etc...
again, if its for your own personal use, u should be alright...
Robot55