I whole heartedly support the suggestion to look up a surplus metals house. They are always in the "industrial" areas of large towns. They buy from the fabrication shops in these areas. No fab shops, no surplus.
You ask about steel but, I've also bought aluminum and brass at these places. Buy it by the pound mostly, although some specialty stuff is priced "as marked."
Some of these places will have piece parts for sale as scrap metal. For instance; about half of the shops I frequented had a bin/barrel full of caster wheels. Another had aluminum, heat sinks--brand new--as scrap.
Don't just do business with just one if several are available. Visit them all, you will get a feel for what type of metals/forms/shapes each specializes in and be able to more reliably get material at the spur of the moment.
You need to visit frequently as their stock changes constantly. One place had a large stock of 1/4" aluminum diamond plate. I kept looking at it each trip, but, by the time I decided that would make good decking for a car trailer, they had sold it all out.
They are great for hobby and one-off projects. But, you'll likely find they won't be able to fill any production quantity orders, especially not repetetive orders of the same stock. You need the jobbers for that stuff.
Drive your truck, bring rope, braces and a tape measure. Wear old clothes and gloves because you are going to load it yourself, get down and dirty and have a ball. There is no such thing as a quick trip to surplus metals store, You've got to look at all their stock.
Dave