Thrihyrne

I would suggest checking Deviant Art and browsing yourself. There is a ton of HP art there, some of it quite good, depending upon one's taste and, to my knowledge, that is THE most commonly used site for fanfic artists to archive, aside from individual, scattered websites. It also provides a way for you contact individual artists without knowing them personally.

P.S. If you want the "big names" you can browse by "most popular."