seat foam kit
Yes, that foam was expensive where I looked as well. I got high estimates from really good shops i know. When i redid the upholstery in my Dart i realized the foam wasn't always the problem but the springs, wire, hardboard, and cotton batting beneath and around the foam that gave the sinking feeling when getting in. they rust and break and the cotton displaces and breaks down.
i made new hog rings and re-wired across where need be from scrap material from work. One spot on the seat frame needed welded with a little patch where i think the rust and weight of passengers cracked it over the last 50 years.
Most of my foam was okay. But where it wasn't i replaced the foam and batting with free material sourced from discarded couches on the curb. looked for the cleanest one, took only the cushions and had excellent material to shape to my needs. it cuts easily with a bread knife. and like another FABO member said it glues well with spray adhesive.
This guy was a big inspiration:
https://youtube.com/@mecaupholsterytips
Even if you dont do the upholstery you can fix the seat structure and save some costs for other issues on your old girl.
Not the best seats on these forums but i saved $2000 doing the work myself.