There are a few solutions.
You can either buy the material and cut your own. Or if you're looking for a specific replacement that you know a guitar company makes - send them an email. Or if you're happy with just a standard sheet of acrylic, then you can get these guys to cut one for you http://stores.ebay.co.uk/Sheet-Plastics-Suppliers.
You could also try asking a local guitar shop if they can, or if they know anybody that can cut you a custom shape. Just supply them some material and the design you want.

If you're confident enough with a few hand tools, I would look for some nice material and cut your own.
Don't you hate it when you're reading a sentence and it doesn't end how you testicles.

Did you hear the one about the boy with the short atten