Replacing door striker rubber inserts

On an '82 MBZ 300SD, both rear door strikers' rubber inserts are worn out. Can I get the inserts some where without buying the whole striker?
Is there a secret to removing the strikers' allen screws? I'm afraid of rounding out the allen screw heads. I put a cheater bar on the allen wrench and I get a lot of torque but the screws won't budge until finally with very much torque it starts to round out the hole. thanks, George
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