I took the same path a few months ago and made my own multiple sets after refusing to pay the prices being asked for a pair of these.
You need to ask for a standard sheet of Number 8 mirror polished stainless steel in the thinnest grade they will handle (some companies will only handle sheets to a certain minimum thickness since they are very fragile and can bend easily if being cut manually via a machine). Of course, if they have access to a laser cutter then that is not a problem. I got them to cut them on 0.5mm thick sheets which is virtually paper thin - but you may need this advantage since many pinball machines have a very small gap between the playfield and the cabinet. If the sheet is too thick it will scratch every time the playfield is lifted.
For that reason, I would also recommend buying a roll of black felt adhesive tape to attach to the sides of the playfield - this will stop any chance of the mirrored sheet being scratched when the playfield is raised and lowered.
B/W standard and widebodies are both the same measurements for the mirrored sheets.
