Yes, you can get the door lock cylinders off of an box-body Crown Vic or off some other late-'70s to late-'80s Ford vehicles (including some trucks and vans). Take your ignition key to a boneyard and try it in different Ford locks. If it fits into the cylinder, take the locks and have them re-keyed to match your ignition. You'll probably need to swap the arms from the back of your current locks to the back of the new ones, so it'll fit the Mustang. If you want the locks in black, be prepared to paint them yourself.
The real problem is if you want to change the trunk lock too. The trunk locks for the Mustangs were very specific to the car, and I don't think you can just change out the cylinder. It's possible you could get the trunk lock off of a pre-'92, police Crown Vic, and have it re-keyed, but I don't know if it'll fit the 'Stang. If you can live with having a separate key for the trunk, you're all set.
There is a reason that those single-key lock sets command such high prices. They're rare, and not easily reproduced.
The real problem is if you want to change the trunk lock too. The trunk locks for the Mustangs were very specific to the car, and I don't think you can just change out the cylinder. It's possible you could get the trunk lock off of a pre-'92, police Crown Vic, and have it re-keyed, but I don't know if it'll fit the 'Stang. If you can live with having a separate key for the trunk, you're all set.
There is a reason that those single-key lock sets command such high prices. They're rare, and not easily reproduced.