My two cents:
1. "New seats" for C-150s are no longer manufactured or available. You must repair or replace-with-serviceable used seats. Passenger-side (right hand) seats are plentiful. Pilot-side seats (left hand) not so much.
2. TN Wings can repair and re-upholster your existing seats, as can others.
3. If you know the specific part number, try
Dawson's Salvage [
aircraftpartsandsalvage.com]. I had good luck with them. Other salvage places may also have something for you.
4. AFAIK, the seat backs from the pilot side and passenger side are interchangeable in most C-150s -- but you must get the correct seat part number for whatever year plane you have. I think. Maybe that helps.