I did google searches & read a bit over a few days & still couldn’t work out what I needed.
I searched just trying to buy foam without knowing exactly what I needed.
I then came across a web site in Australia called “home upholstery”. I got onto their “cut foam calculator” on the web site and still couldn’t work it out, so I phoned them & they walked me through the process and told me to select “transportation seating” as the intended use of the foam & that narrowed it down to 3 selections. I went for the medium firm one.
Plus delivery was only $9, it turned up in a couple of days.
Still waiting on the seat cover from the US & my mate has the seat base under repair, then I have to work out how to best shape the foam & fit the cover.