The bike is rather new yet, especially in numbers of units sold, but that is picking up now that pricing is better. Vendors with farkles for it are just starting to pick it up also. Russell Day Long Saddles can re-make your seat to your specs but that requires sending the original seat in to them and they use it to build on. Spencer seats also does much the same for less $$$ (can find his contact info over on the ST-Owners forum as he is popular over there). Don't know of any other seat makers with aftermarket seats for this bike... yet.
There is a member (a few actually) here who fitted a Utopia back rest on his. He decided he needed a better seat (the Russell Day Long) so listed his for sale in the Classifieds forum for CTX1300 Parts HERE. Send him a PM to see if he's sold it yet. If so, or if you prefer, there is link there to Utopia for their new product developed from this member's seat. There are other threads also that show how to install it.
Lots of information in the various forums here so please be sure to browse around.