Industrial metal supply company. Since you're on the other side of the country, I'm of no help. Out here we have Industrial Metal Supply , Patton's Steel City and several smaller companies. You might have a Patton's out there though.
I've bought from mill suppliers, industrial hardware vendors, etc. and as was said the price will be a fraction of what you will pay elsewhere. Square stock comes in 20 foot "sticks" but in my experience the supplier will cut them in half for a small charge so they are easier to haul. If you ask at your local welding supply store they can probably recommend a good vendor.