Perhaps one of the cheapest sources of scraps would be buying an old frame and tearing it down.
https://chicago.craigslist.org/wcl/b...839185698.html
https://chicago.craigslist.org/chc/b...853238022.html
If you have a local bike co-op, you can ask them if they have any scrap frames that you could tear down. With some luck, you could find something for FREE. Search the curbs for bikes left out for trash? Free stuff?
Shapiro's in St. Louis has a lot of basic round chromoly tubing. But, you may well also have local steel stores that would have remnants. Although, I wonder if Shapiro's is tied to the aircraft industry.
KVA Stainless used to sell practice remnants.