Sadly I dont know anything about rubber manufacturers as I only really have contacts in UK. And I think pretty much all rubber is foreign manufactured, eg the Dunlop plant is in India and the ICI one in Malayasia. I dont think there are any small concerns (which are the sort of people I know) which make rubber products as you need pretty heavy industrial equipment to form rubber, and then another huge factory to vulcanise it
That is why polyurethane is so popular these days - you can pretty much mould it in your kitchen.
Perhaps a step beyond tippex would be to buy rubber sheet (70 or 80 shore), cut to shape, and get some white vinyl letters cut. This would be pretty much as good as the OE parts and about £40 cheaper.
Cheers
Jules
RatSport www.ratsport.co.uk