If you have some engineering and design skills, as well as basic metalworking tools, here are a couple of places to seek inspiration.
First I would suggest finding a good Marine Chandler who supplies stainless steel yacht railing and stanchion components. Most use 25mm or 1" inch tubing. Thick wall versions of the tubing need to be bent using a hydraulic press, so they would easily provide sufficient support for an application such as an exam table. Most Euro-style Gyn chairs may be built using these components with stainless tubing. Although one would need a hydraulic conduit bender (most places like Home Depot or Lowe's rent them), you could assemble a tubular framed Gyn table using little more than a hacksaw and Allen wrenches without any welding required. Although the finished product would be of very high quality, quality does cost somewhat so it's not a low budget project.
Secondly I would check the website for Lee Valley Tools in Ontario, Canada. If you follow the links to woodworking; jigs and fixture parts, you will find a wide selection of knobs and other hardware for use with threaded parts, as well as a number of high quality adjustable metal track systems. Elsewhere on the website you will also find other useful hardware such as Accu-Glide drawer slides. Lee Valley sell extensively by mail-order and the package is always marked as "tools and hardware".
If you want to learn fiberglass fabrication skills you can consult any boat repair or fiberglass DIY publication for a basic guide. A pair of knee crutches could be easily fabricated with a starter kit on the back porch one sunny afternoon. You could mold the fiberglass around a plywood core used for attachment of the metal swivel, and then finish it with a pour-over finish such as that used for Decoupage craft projects (colored with fiberglass Gel Coat pigment). The result will be near the same as a professional version.