I was able to get a gasket (for old Casio watch) replicated by Marco Rubber. I sent them an original and they took measurements and sent me 200 of them in return. It cost me about $100 IIRC. It's one of those deals where the unit price drops steeply as the quantity increases. Basically, it's about $100 of effort for them to make any.
Of course the watch gaskets are a lot smaller. They also have several materials available. It might be worth getting a quote from them. I think they charge for the effort to measure the old one -- stops folks from using them for free measurements.
www.marcorubber.com