I've replaced some troublesome side mirror guts and added a whole mirror to an early no-mirror car with junkyard replacements. Most I've ever looked at or found seemed to be all okay unless broken. I would check any of the local yards and look for a donor. You may luck out and find a better gasket than the one you have too! I've reused many and not really worried. You might even find someone local to get one cheap too. Local Foxbody group on Facebook?.
I've also made similar gaskets from closed cell packing foam:
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