RoadWarrior;
I can't tell you what aftermarket seat to buy, because I haven't tried any. but, I do have this soft bottom gel pad. Mine is the double thickness touring cushion. I have ridden my 07 rstd for long distances and have no problems. there is no vibrations when using this pad. here is a link.
http://www.geltechnology.com/
They aren't cheap but alot less than a new seat that you may not be happy with.
JIm Bell
Lexington, Ky