If your town(or a surrounding community) has a reasonably large agricultural, manufacturing or processing plant base as mine does, it's a safe bet that you may have a wholesale rubber supplier in the area. Industrial plants are full of one off custom hoses, gaskets and belts that must be replaced on little or no notice. Check the yellow pages, they are typically one or two shop operations at most in size.
My local guy has never let me down and I have brought in stuff that everyone swore was a factory only item, they have had it in stock or duplicated it in the same day if the circumstance called for it. The prices for these services can range from surprisingly cheap to out of this galaxy(dependent upon your schedule). Rubber port hole gaskets(regardless of shape) should be no problem for any rubber company worth its salt.