What are you looking for exactly? Just a reupholstering, lighter weight, better support? There are many things to consider when looking to replace a seat. A lot of people just shop the local salvage yards, but be careful with seats, as they might have been water damaged. If you do look on the used market, pull up the seat material off the foam padding and it should have no water marks, large stains, etc.
If you want aftermarket, one site that I have on bookmark here is Welcome to LeatherSeats.com
I had one of my older cars reupholstered with Katzkin covers, they were awesome and if anything else, helped with resale value Katzkin Leather Interiors