I want a custom seat
If you, like many other 250 riders, find your seat somewhat less than comfortable after a few hours, you can have the seat modified, or modify it yourself. There are also numerous businesses which will reshape your seat for you. This is an incomplete list: Sargent Cycle, Rich's Custom Seats, and Corbin. Sargent and Rich's will do a custom seat by mail-order, and Corbin now has a ready-made seat for the 250, the only company to do so.
New seat cover
If you are looking for a new (non-OEM) cover for your stock seat, look at long-time N250RC member Custom Seat Creations. She has lots of choices.
Praise has been universal for the Corbin from those club members who have bought one. They are really liked by the long-distance fraternity. The Corbin seats are firm & provide excellent support.
Rick Mayer seat
Rick Mayer Cycle makes custom, handmade seats for bikes. They do mail order, or you can ride in to the shop in Anderson, CA. The staff is pleasant, and you can watch the work progressing.
Rider comfort is better than with the stock seat, but the great improvement is in the feel for the passenger. Reported comfort was better than the pillion on a friend's Corbin.
The work done by Rick and his crew is absolutely top-notch, and the seat looks quite natural on the bike.
Custom stock seat
Another alternative is to get your stock seat reworked by Spencers Motorcycle Seat Mods. Spencer re-works your stock one, so it has the same pan and cover. You pay for shipping both ways, and it's either $50 for the foam only or $75 for new foam and orthopedic gel. This gel is solid, like the stuff in better bicycle saddles, so it doesn't go away where you need the padding most. It looks just like the stock seat but feels much better.