There's a member on MonteSS.com called Clutch Grabwell. He's done some work on the interior of his car. He modified a Caddy Eldorado rear seat to fit his car (he said it was alot of work), and he's using newer GTO front seats. I've heard of some people using seats from newer Monte Carlos (year 2000 & up). I changed the seats in my Monte about a year ago. I used a backseat from an '84 Buick GN with the Lear Seigler seat option (very hard to find), and I'm using front seats from a '96-'98 Pontiac Grand Am. I had the seats re-upholstered to match the interior of the car in a combination of cloth & vinyl, and had an "SS" logo stitched into the front seatbacks. I think it turned out pretty well. I inquired about using leather when I had the seats recovered, and the shop owner told me that the cost of the leather material would add about $700.00 to $800.00 to the cost of the job. The vinyl material that they used has a grain in the material that looks very much like natural leather, and I've had a few people mistake it for leather.
Most GM bucket seats that have been produced over the last 30 years, have the same mounting bolt pattern for the seat track mounts. There are probably some seats that don't, as there are exceptions to every rule. When I was shopping around for seats for my Monte, I tried seats from an '89 Firebird, the same '84 GN that gave up its rear seat for my car, a seat from my daily driver 2004 Cavalier, and the Grand Am seats, and all of them bolted directly to the G body bucket seat tracks. That does give you a fair bit of choice, without having to alter seat mounts, to get them bolted to the proper G body seat tracks.
Hope this helps. Good luck. Let us know how it works out.