Quite a few seat specialists on ebay and normal web channels sell adapters similar to what you seek. Do some searches on ebay 1st and then check Automotix, expanding your search to specialty and custom seat vendors or interior only shops.
Oh, we need some front and side images of the seats. Rise, depth, length, width, etc. I know Phil W. (Dr. Phil on our site) installed some later model P-car seats he bought from a West Coast rehab group. he had to custom mod them and then read here or elsewhere afterwards that there may have been a kit solution or similar.
Good luck...