yes oil barrels are good steel. gas or avgas barrels would be better yet but are way harder to get. normally you can simply look in the classifieds and find folks getting rid of barrels. Oil change places are a good bet, docks and shipyards tend to be good, airports, machine shops, bakeries, all kinds of manufacturing, train yards, &c &c. sometimes a scrap yard will have good barrels but not often.
You just gave me an idea! There's an airport in Klamath Falls which includes an Air National Guard base, and I happen to know one of the supply sergeants from the base -- I'll ask him if he knows what they do with empty avgas barrels! (If they ever get it in barrels and not by tanker truck!)