You can buy brand new Carrera calipers here for a great price:
Pelican Parts - Product Information: 911-351-425-03-M4
Pelican Parts - Product Information: 911-351-426-03-M4
By the time you buy used Carrera calipers and rebuild them you will probably not save that much.
Again, for a track only car I would pull the rubber seals off as they will just burn up.
Here is another option that some like:
Porsche 911 Big Brake Kit.