I purchase mild welding steel from 'Ripley Metal Works'. They are located in Ripley, Ohio on the banks of the Ohio River across from Maysville, KY. They have a scrape bin and the manager of the plant will turn you loose to rummage through it and pick out what you want. When you are finished, he surveys it and gives you a price. It is very cheap.
He took me on a tour of their plant. It is impressive. They use computer technology to cut and fabricate complex metal projects. He told me, the company is owned by a couple seniors. He said they work on a knife thin profit margin and many years only meet cash demands. The owners tell him the only reason they keep it open is for the jobs it provides in the local economy. I have to wonder since they only fabricate and buy their metals - steel, aluminum, copper, etc. - if they will stay in business.