Local metal supplier and cut a chunk of bar stock of the metal of choice, or some 1/4" wall square/rectangular tubing.
Surely something usefull at HF or similar store... A nice wooden box of number punches, a mini anvil, a stainless steel box (filled with rock, nuts, screws, ball bearings, sand...) maybe even a numbered drill set if its container is suitable manufactured to give you a flat bottom of the correct size.
Fancy kitchen supply place and look for a marble container of the dimensions you want.
If you have time to kill, local landscape stone supplier (one with large piles of materials to pick from) find the flatest and wear it flat on the sidewalk (or kitchen counter top maker who'll run it on his stone wheel). =-)
Alex.