Hi I just emailed some info on where to find arm boards. But frankly it is not that big of deal. It is either 1/2" or 1" thick piece of black acrylic. You have your existing one for a guide for size and screw holes. Get a Rega mounting template for the arm hole location. All you have is the one hole. You could also try this out of Corian or wood.
WE have plastic houses that have scrap that they will cut to size open to the public, or a countertop supllier for Corian
WE have plastic houses that have scrap that they will cut to size open to the public, or a countertop supllier for Corian