The seats in my S2a 109" are I think from a Maestro or Montego. The seat cushion though angles up quite a bit they are ok for me 6'3" but SWMBO (5'4") doesn't like them as they lift your knees and make reaching the pedals harder.
Other common ones are Metro, Skoda, Volvo (340 IIRC), Manta GTE (Recaros). But anything where the runners bolt down to a seperate subframe should be quite easy to fit, I have in the past looked at I think it was Citroen BX and thought they looked suitable.