Mike, I buy all my steel and aluminum for my various projects here locally at a dealer called Tigrett Steel. I'm sure there are similar places all around you. What you should ask for is a piece from their "drops." These are pieces that have been cut from larger jobs and end up in their scrap pile. They will usually shear it to size for you for nothing. Anything from 1/8" to 1/4" thick is sufficient. The mounting holes for the bracket itself are in an approximately 3.5" x 4" rectangle so the plate will need to be somewhere around 6" square, again this is not critical, it just needs to be larger than the mounting hole pattern. If anyone can't find this material and make it, I will be happy to fab it up and ship it to you.