You might be able to get the parts you need out of a 4 door car. I put '69 bucket seats in my '68 ragtop because I wanted headrests. I had a pair of seats but the headrests were gone. I have a '70 Polara parts car with a bench seat and headrests so I pulled them out. The posts were the same distance apart as the B'cuda seats openings. I did have to cut the posts off a little since the were about 3" too long and wouldn't go all the way down.
The whole point is I think you could find the parts in a C-body or other more common car. I believe all the bucket seats have the provisions for headrests. You will need the insert that goes in there (plastic IIRC) and the chrome bezels on top.
Hope this makes sense.
Dallas
missourimopar.com