So I noticed that the Google Ads on this page often run a link for Splash Page Comic Art. I've actually bought from them at cons more than once. I purchased work by both Jock and ChrissCross.
So yep, I'm give them an unqualified thumbs up. They rep a number of great artists, know their stuff, and their website's clean and easy to find what you're looking for. In my opinion their prices are pretty fair, but I'm not really in the know so much anymore.
Check 'em out.
www.splashpageart.comBy the way, the above piece is by Chriss Cross and the original is for sale on the site.