A steel mill typically, at least the products my company uses, does a run on the order of 20,000 pounds of steel. The problem you'll face is a distributor may order from multiple mills, based on who is cheapest.

To help with this, request "material certifications" and should be free of charge. A mill will produce and supply them to distributors and it should contain material composition, stress/strain data, etc. This will at the very least give you a more accurate carbon content, not a range like in a ISO/ASTM standard.