Plenty of these type frames come up on eBay Freaky. Often they're as cheap as chips. I found my latest project, a super rare Hindall framed Ducati 350 (below) for a song. Over the years I've bought a Metisse, a Champion and a Trackmaster frame on eBay for less than a grand. In fact the Trackmaster only cost me $145us. It was covered in surface rust and the seller didn't know what it was. I did and got one of the bargains of the year.
Check out ebay item#180147063104, a Trackmaster Bultaco frame that only got bids up to $203 but didn't reach reserve (probably $500) and eBay item # 220138130482, a '75-77 Bultaco Astro frame that sold for a pissy $295.80.