I would suggest talking to an upholstery shop or to either MII Specialties or Second Gen. This is an item that will require some expertise to locate.
TMI (http://www.tmiproducts.com/) makes seat foam and upholstery for the early Stangs, but the @#$!'s don't make anything for our cars. You should register a complaint with them.
I'm curious how close a '65-'66 seat is to a MII seat...its been years since I sat in one of the early seats, but they certainly "look" a bit like ours...other than that chrome side trim anyway.
The reason that I bring it up is that TMI has a new seat foam and upholstery set for the early Mustangs that has pronounced bolstering to hold you in the seat better. I could sure use that in my MII!
You may find that the 65-66 stang buckets could possibly be too wide. For chits and giggles I'll have to measure my '66's buckets and actually see. I've about had it with those plastic racing seats in my II and am replacing them with the Pinto hi-back seats like in the 69-70 stangs, only narrower just for the II!
The seat in my '66 is approx 21-22" wide as it's widest point of the seat bottom. I didn't measure the seat track spacing, figuring it was wider than the II's.
Another seat to consider is a set out of a pinto. I'm on a mission for a pair of hi-backs similiar to those used in the '69 deluxe and '70 std. interior mustangs. Some Pintos came with these and i assume they'll fit just the same. Will find out soon.
The '69-'73 Mustang high-back seats do look a lot like the mid '70's Pinto high-back seats. You'd have to do a bunch of measuring to see if the Mach 1 seat foam and covers would fit though.
I finally found a decent picture of the "sport" seat foam and covers that TMI offers for the '65 Mustang: