NCs are getting older and in many cases the upper spring isolators are in rough shape and not ideal to reuse when doing a suspension swap. These isolators are a single piece of rubber that not only cushions the top of the spring, but also extend down and function as a dust boot as well. You may want to consider changing these out when doing a spring/shock change.
For the rear springs on an NC. Mazda p/n NE51-28-012C. Sold individually, one needed per spring.