Check out my how-to at YZF600R-USA.com. Go to How-To's, and look for Reupholster your stock seat.
Simply, it is not very hard to remove stock cover, add/remove foam to customize the shape and staple a cover back down. I have even found other fabrics at fabric store and reused the stock cover when only minimal changes were made.
For instance, I added foam to raise my seating position about 3/4", levelled out the front so it didn't smash me into tank so badly and moved the "break" in my seat forward so I could rest my but against it easier.
This stuff costs big bucks at custom shop and you can do a very nice job yourself. If all goes to hell, you can play with the shaping, get that right and send it to someone to finish it off with custom cover or whatever. Nothing would be worse than spending $400 to get seat redone only to find you don't like the shape.