Just a thought
Just in case you haven't gone down this particular route, I suggest you look to the scrap merchants for such a frame - the specialist breakers maybe.
Back in the days, a frame would not be scrapped but put in a jig and straighened, for reuse.
You will understand that a "cheap frame" is not likely to arise because the frame number is the main route to registration for highway use, along with the engine number.
Perhaps one from a bike that has been stolen with the frame number damaged/erased could be found, but that is not likely on here