I think pretty much any small P/U seat will fit. You're still going to have to build a bracket for it. Check out junkyards for Comanches, Cherokees, Rangers, etc, just to name a few. You can probably save a few bucks. My bench cost $30 from a Comanche.
Go Fish! <*////><
But the right word at the right time... "Hey, give me a little hug!" That's the difference between lightning and a harmless lightning bug!