Post welded? You mean pre-welded? Yeah, you can buy frames. As far as seats go, anything goes. Make 'em outta plywood and foam. Rip 'em out of a honda. Tractor seats. Metal-framed plastic stacking chairs with the frames removed. High-buck racing kart seats. Hamemade bomber seats. Lotsa choices!
where do you live? here in canada they have some sweet bucket seats for like 150 each at crappy tire, if you live anywhere else, i would said check amazon.com