I'd say try looking at the local hobby shop for some sheet styrene or plasticard. It's usually used for model railroads. The thickness will depend on how embossed you want the design to be. Using a sharp knife or a fine-toothed saw, carefully and slowly cut the card into shape then glue it into the press using plastic glue. Let it cure for 24 hours before using, just in case there's any fumes or such! Or if you want to be on the safe side, use carpenter's glue.