Its basically impossible to find a signiture. You can go to
www.google.com and use the image search to find an avatar. If you want a sig, I suggest asking someone to make one for you. Three guys that I know that are really good at making sigs is Crono_Link, Dyne, and Rndm_Perfection. The topic that you would post on to request one would be this one:
http://www.gametavern.net/forums/showthread.php?t=6120