I will chime in here with what I know. Lord knows I have answered enough PM's about this at this point, take everything as with a grain of salt. I may be wrong, but passing along what I know.
As starpc said, Mike is his art rep. You can certainly set up a commission through him. Commission, not sketch.
Eric will do two types of work. Sketches and full blown commissions. If you want one of the later, contact Mike and set it up but be aware that artist charge a fair amount more for full done work then they do for covers. The other way is to get a sketch from him at a con.
Eric does not make anything precon, and does not take a list. He shows up and does sketches in either pencil and ink or pencil ink and copic depending on what you want. If you are getting a sketch from him you wait in his line while it gets done. You can't drop your book and leave and come back and pick it up. He takes a fair amount of time with his sketches, so he gives it to the fans that are willing to wait. If you are getting a sketch at a con, that could well be the entire day.