If you can’t make it to conventions, some artists do sketch requests if you get in touch with them through their websites. I know Todd Nuack does. He charges a lot, like somewhere between 80 and 150 for sketches.
http://wildguard.com/index.php
Dynamic Forces usually has some pretty good deals for sketched covers. I got a blank Dark Avengers sketch of Spider-Man by Romita Sr. from them. That one I paid a lot for, but I’m getting a Venom sketch by some artist for only 20 bucks. I think you can still get a blank Dark Avengers cover with a Captain America sketch from them for 20.