Loads of threads on here so have a search.
Mk1 R5 is easiest - I've got these in mine but R25 seats also fit. If you are a bit more creative then Audi A4 / Seat Toledo / Leon / Volvo 300 series can apparently be made to fit.
If you want foldability I'd recomend the R5 seats though - mine will recline pretty much flat but because they are from a 3dr R5 also tilt forward.