like... a belt?
could be fabbed?
I think for stuff like that, it'll be cheaper to just buy it. You wouldn't fab your own screws or bolts, and i think for some of the little stuff it's the same way. McMaster Carr is a huge supplier of parts for anything and everything, and I think they'd be a good place to go. You can fab some bigger structural parts at home, although personally if I were going to want to make one, I'd want to make it as simple to build as possible. Alternativly, you could make fabbed parts and then sell kits or something.