You may get lucky and be able to find a pin in the help section in the auto parts store. They have replacement pins for the other doors. They may use the same size on the tailgate as some other vehicle uses.
Worst case, you could have some spun on a lathe. Don’t think these would be too hard to copy. Or as Phil suggested, maybe find pins slightly larger and spin those down to required dia and length.
Of course, having the correct part numbers and getting lucky from a dealership would be best. Have you tried Dorman manufacturing? I got my fenders, floor pans and body mounts from them. They might have the pins and bushings you’re looking for