Both suggestions above are good ones. I went the Oregon Aero route and love'em. The seat comfort outlasts both my personal endurance and my fuel supply.
But if you're a "roll your own" kinda guy, dealing with seatfoam.com folks is a real pleasure as well. Great materials and wonderful folks. I've personally known Jim and Jan for over 20 years; really good folks to deal with.
If the Oregon Aero price makes too big a dent in the budget, you can always do only one seat. That's what I did; made the LH seat (my seat) top notch and let the co-pilot have a perfectly good cushion with less expense. No ones complained yet. Besides it only get used less than half the time.
Bottom line, don't skimp on your seat cushion in the comfort department. You'll appreciate it more than you think.