As I'm sure somebody else has already said, going to the local Goodwill or Salvation Army stores was what I did for years to find blast plates for my own club. Having a drill press to drill the right sized hole thru old rusty cookie sheets, stockpot lids, and steel trays helped a lot. Occasionally, they'd be a burn thru, but not that often. And the bent edges helped to maintain their mostly flat shapes. The biggest complaint that I got was, "They don't look very professional." Personally, I figure that function is more important that looks. My wife would agree with that every time she has to look at my face! But that's a whole 'nother topic. These makeshift blast plates from second hand stores worked great for 1/4 to 1/2 in rods and smaller rails.
When we got into larger rockets needing much larger pads, we went with worn out farm implement discs. Occasionally, we still use them under very large motors to keep from making too deep of a hole in the ground from the motor blast. Gee......., I wonder if that's why we call them "BLAST PLATES?"
Brad