NG8264723, I believe there are some repo offerings along with NOS parts which should make finding what you need pretty easy. You can find them on E-Bay and various part vendors by searching by part number. The door lock retainers are D1ZZ-6522023-B (replaced by "C"-both numbers show when searched). Since your car is a 73 it won't be involved the mid 72 change on the trunk lock, so you will need D2ZZ-6543629-A. These retainers were used on other Ford lines so availability should be pretty good. (Sure don't hear that much for 71-73 parts)!
Don @ Ohio Mustang Supply would be a good place to start. Don (OMS) or Mike @ Motor City Mustang probably also have used ones.
Good luck on your search.