I just purchased a 82 300D. Would you believe it only has one ignition key? Where would be the best place to get spares made besides a dealership? Can I buy blanks and have them cut? Help

yes you can.

The best place is your mercedes dealer. The former owner of my 450SLC bought some blanks and took them to a hardware store to be cut, they didn't work. I went to my local dealer with my VIN number, registration and license and got the correct cut keys in about a week. The parts person told me they send the info down to NJ and the keys are made there. Even with a vintage Mercedes the dealer will require proof of ownership and these folks know me well.