for all my avatars, i go to www.avatarity.com or www.avatarhell.com (registration required, but free)
otherwise, i've got this really cool site with a huge base of anime: http://www.torancastle.net/Avatars/
this one's really, REALLY good. it's got a huge collection of avatars, just select one in a category (it's alfabetic)
all these are eather 50x50 (almost all of them) or otherwise you've got the "special avatars" (mine's one) 100x100 with cool designs.