The junk yards always keep leather covered seats if the leather is any good at all because fresh leather runs $1000 a seat.
I scored a seat from a local junk yard for $200 just to get the leather. I pulled off the bottom cushion and the leather off the back, held on to the seat belt buckle and the seat control switches, and tossed the rest.
Convertible and coupe should use the same seats, right? It would be good if e46 seats would fit because e46 seats are softer and shaped better and they don't 'pop' when you corner, but I think the e46 seats are wider, sized to fit the largest common denominator.