Try Pinky's auto which I believe is still on Capitol drive. They specialize in used Volvo parts and they're local. The problem with changing seats only however is that they won't match the rest of the interior as is unless you also replace the rear seat cushions too. If not done right, it'll look "rigged" and when you want to sell the car someday, most people would wonder why the seats don't match.
IMHO, the cloth seats are more comfortable and wear better than leather (unless treated often and few people do that).