If you have a surplus center in your area, that is probably the cheapest. All metal has really gone up in price in the past year.
When I started buying iron at this one local surplus place years ago, if you wanted a 6" piece of steel, they would hand you a hack saw and send you to the pile. Now, they have improved. They will make one saw cut for you free. A lot of time, I buy a 20' stick and have then cut it in half, so I can haul it in my pickup.
I also found a place that constructs metal buildings, and they will sell stuff to individuals. Theirs is new though, and priced accordingly. But if the surplus place doesn't have what size you need, you are in a bind.
Also, for small quantites of special sized stuff, I've order from Onlinemetals.com. There are other places as well.