i shopped for a variety of seats before settling for the HD Badlander seat. The aftermarket companies build really nice seats too. The main difference i found was in the closed cell foam used in the badlander seat vs. the open cell foams used in the tall cushy seats. The seat is firm but it feels consistant even after a 300 mile ride.
There are many aftermarket seats out there, but you have to decide what it is your looking for. Narrower, lower, gel pads, etc...
Mustang, Corbin, LaPera, Saddleman all make good seats. I prefer the Mustang I have, but it raises you off the ground and forward about 1 inch. It is wider in the front and more rounded. It has good fit and finish and a very respectable price.
Try the websites for the different manufacturers to see what they have to offer. Good luck.