https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ebay.com.au/b/Akebono-Car-and-Truck-Caliper-Parts/33563/bn_7014999565&ved=2ahUKEwjc9ez7os_mAhUFxzgGHftOCP4QFjAAegQIAhAB&usg=AOvVaw0jF8K5OJ5rww0Yig_YEipL
Or if you want reconditioned ones, can get them from wreckers local etc as well
https://www.summitracing.com/search/part-type/brake-calipers/make/nissan/model/370z
I have a set of the rear powerstop ones, you can buy them from a few places.
Rotors are off the shelf and a common part, unsure on how you would fit them to an r34 but the handbrake size is the same
I wouldn't buy new gtt calipers from nissan they will be expensive, get a second hand set and throw a seal kit from someone like tarks and give them a paint
Alternatively
https://www.mrpltd.co.nz/product/skyline-r32-33-34-big-brake-kit-355mm/
https://www.mrpltd.co.nz/product/skyline-r32-33-34-big-brake-kit-324mm/