Seats come up all the time on Ebay and on this forum. Unless you really want to craft your own, I would put it out on the forum that you need a seat. I presume that you probably don't have the mounting hardware as well. As well as OEM which include a couple of style eg: Comfort, Std, you have a choice of after market custom, Corbin, Sargeant etc. And you have heaps of variations the customs it can be a bit of a nightmare trying to find one that suits. I bought a spare original seat and had it shaved, refoamed and recovered to original colour, which was great until I lost a heap of weight, ( mainly of my bum ). So I now run a gel pad on long trips. If your seat is not perfect you can have it rebuilt, run a gel pad or an air pad, and lots of these pads can be fitted internally.
Regards Martin. :riding: