My yet to be delivered order of 4130 tubes (for a rack) is coming from Stockcar Steel. https://stockcarsteel.com/ Not the best range of diameters and walls but good prices on what they do have.

My current rack project uses 4130 through out. 3/8" x .035" for the perimeter hoop and the center strut on each side. 5/16" x .035" on the rest. Andy