Sounds like you are building a big BBQ grill or smoker.
I'd suggest going to a scrap yard and looking for something that's been tossed.
While you're there, you might find lots of other goodies for yourself.
If you find what you want at a scrap yard, you typically pay for the weight of the steel. Customers, on the other hand, should pay 2 or 3 times what you pay at the scrap yard, plus your time in hunting it down.