Luckily in the hornets any 70-up hornet, gremlin or concord seat will fit -- you might get just some crap seat to drive on until the slowpokes get you seats done. They bolt on with four 5/16" nuts over studs -- easy except for the gymnastics to get you arm under there!
I paid $16 for a pair of usable (but worn and ugly) seats from a Concord. I never used 'em -- too bad you aren't near me, I'd give them to you. They're in your favorite color... I think the AMC brochure calls it "Shit Brindle" :-)