I'm in the same boat. I plan on trying my hand at making a seat from fiberglass and a piece of sheet metal. At this rate I probably won't get to it for a couple months.
A while ago I made a simple base from sheet metal that just slipped on and it worked pretty well (except I made it too snug and it chips off the paint). I will try to find it and take a picture to show it's look. I also tried to upholster it myself (HAHAHAHAHA) and failed.
Like Jeff said, the OEM seat is uneven and has wires running under there. Plus it is secured to the bike using two wingnuts inside the box on the back of the bike, so you would have to reroute wires, chop the seat, reupholster it, then figure out a new way to secure it to the bike. Not impossible, but it will take some work.
As far as aftermarket seats there aren't any that I've found that will just bolt on without some sort of modification(s).