Why exactly do they need to be replaced, is it just worn leather, or are the seats bad (like you mentioned about the passenger seat) if the leather is just warn, you should try some soft scrub (the lemon type with no bleach) and a wet towel, work it into the leather real easy, and keep it out of the stitching, then wipe them down with another wet towel, dry the seats off and apply a good coat of conditioner. I've seen this do literally wonders to old leather seats, and its a real old detailers / used car dealer trick to make leather seats look new. Give it a try. You'll be amazed at the dirt that will come off.
Keep in mind, this is not a regular cleaning procedure, this is for badly worn and discolored leather.