how much does a rear end cost new or used and were could I purchase one on 1990 Nissan 300ZX

It is making a sound and have replaced the bears

Asked by for the 1990 Nissan 300ZX
You can find out whatever you need to know by calling an auto parts recycler (junkyard), they have prices and a crossover list for you. Prices are so variable, I can't really give you good information from here.
If you want to find a specialist, here's our directory link for you: