You might want to see if there are any small metal fabrication shops in your area. At my place of work we have lots of sheet steel drop off in this size. Like my company, they might sell it to you for the scrap steel price. I am also looking into building one and thinking of box bending the piece (all four sides) to help keep it straight. If you take the fab shop approach you might ask about this as well. For me it will probably cost a 6 pack to have one of the fab guys do it after shift.