Check the handbrake cables before you start sourcing calipers. I had an absolute mare trying to get The Bitch through her MOT, she kept failing on the OSR parking brake being well under minimum requirements (16% of the weight of the car applied across both wheels. About 140kg's for the GT) at 30kgs. The nearside pulled 120 on its own. I changed the OS handbrake cable and that cured the problem, both sides now pull 120kgs.
Cables are £50 a shot from Toyota. Take it off and check the condition of the seals. If they're perished and it looks like the cable is going rusty or a lot of water pours out of them, they're mostly fucked.