http://bestcovers.cheatman.net (newish site - lots of new covers being posted)
or
www.cdcovers.cc (older site but still reasonably good)
They are the two that I use, I think there is also something like (dont quote me)
www.coversearch.com or something like that that lists a few cover sites. Try (without sounding rude) using a search engine like yahoo etc.
To answer your next question :
www.coverxp.com for excellent cover printing software.