Hey dude Ive installed a few rear z brakes on a few of my s chassis and I always got them from advance auto parts, frsport and ebay.
http://www.frsport.com/OEM-Rear-Brak...FYpbhgodwCQA4A
http://www.frsport.com/Nissan-41090-..._p_281497.html
That was a quick 3 second search in my google bar, im sure you can get them locally for cheap and quicker.
Advance has them for sure cause thats where I got them from for my silvia late last year