I've seen Daihatsu Charade Turbo seats used in a Mini - looked like they lived there.
Gemini, RWD Corolla and mid-80 Tercel look like a possibility too.
WRX seats would be nice - they have the harness holes. But I'm sure wreckers would want a squillion dollars for them!
My Moke currently has Daihatsu Scat 4x4 seats - not great but decent Moke seats are extinct and there is no room for anything much wider.
I bought a second-hand Recaro recliner drivers seat a few weeks ago for a bargain $15!
Even the trim is good, it's just dirty and missing seat runners and headrest. I'm not sure yet whether I will try and squeeze it into the Moke or fit it to the van instead.
Watch out if you get Excel GX seats - the ones with height adjust and lumbar support. I had a 97 Excel GX for four years and 100,000km. At least three times I had problems with the height adjust not raising evenly side to side when the knob was turned. So check it carefully!