A few years back, I was looking at the Atomiczombie stuff and found it was decent plans.
My other crazy passion was flying and building a high wing airplane. Hence oxy/acetylene welding, 4130 aircraft tubing. You can braze 4130 tubings and have a robust joint.
Bicycle tubing is double butted (outside the same diameter, inside diameter shrinks down and ends) for a sturdy frame but even using plain old steel tubing is excellent for a prototype "does it work."
designs.
The cheapest way to parts is as Atomizombie mentions, get old scraped bicycles and cut and weld your own.
For rapid prototyping, carboard and hot glue guns are fast. Does the linkage work, does it bind, range of motion? Once you have a working model, make it from metal and go to town building.
Thankfully my kids were at my elbow learning skills. I had read about Caroll Shelby oxy/acetylene welding aluminum. Couldn't get the hang of it. A guy, Kent White - aka Tinman - taught four-day classes, so grabbed my son and we went. Later, another son went to his classes and works as a fabricator (He also attended the two-week bicycle mechanics training course in Ashland) These days they are fabricating all sort of things, including mountains bikes.