I agree with Roadking - Finding a decent set of used seats will be tough. Anything you find is likely to need some attention and shipping will be costly. That said, I've used 'Franks' (FranksPontiacParts.com) for a few used parts for my '81. They are reputable, but as with most used parts business the inventory is always changing and pricing varies. They are pretty good about their part 'condition' listings. So, if you are looking for used parts online, that's the one business I've had a positive experience with.
You could go with aftermarket seats for $200-$500 each and I think you need to purchase 'mounting brackets' to match up to the F-Body floors. The ones I looked at real quick were from TMI, listed in the National Parts Depot(NPD) catalog, the seats are around $200 each and the tracks are another couple hundred bucks. This is probably the 'easiest' option. But not the least expensive.
If you are looking for stock appearing seats, and this isn't a daily driver (ie. you can take some time to do the job), I'd suggest starting by pulling the old broken seats out to disassemble them. Then, based on what you find inside, you can replace the bits and pieces you need. For example, it sounds like at least one of the springs is beat, you can get just the springs for 75.00. The seat tracks are now available in the aftermarket...around $200.00 per seat (you might only need the driver side, which is what I found in mine). Similarly, both my seat foams appeared to be 'bad', when I took them apart I found only the driver side really needed fixing. (There is foam sandwiched within the upholstery separate from the main seat foam. If you are seeing tears through to foam, you might just be into that first upholstery layer of foam.) The main foam could be repaired in sections, or you can buy a full new seat foam.
Going through and fixing just what you need is probably the least expensive and it would be my personal recommendation for a weekend driver.
I disassembled and recovered the seats in my '81. By this time I'd consider myself pretty 'advanced' at DIY on these cars. But I had never taken a seat apart before I tackled my seats. Like Roadking said, it's not a beginner project. But, if you are relatively handy with tools, have patience and can follow instructions, you might be surprised at what you can do. There how-to videos (like on Legendary Auto Interiors website) and you'll find several people here on TAC that would be happy to provide guidance and advice to get you through if you go the 'do-it-yourself' route.
Either way, let us know what you decide. And...Post pics!
