The ball joints in your front upper control arms were not designed to be replaceable, although there are some options available. They'll last a long time if they're kept free of dirt, but if the protective boot cracks they'll start to degrade. Unfortunately, the rubber boot doesn't live forever. This replacement boot can be installed easily to replace a cracked one, and may keep you from having to buy a new upper control arm or press in a new ball joint.
If your stock boot is damaged, check to see how freely the ball joint moves. A good ball joint will be stiff, a worn one will move easily.
Genuine Mazda part NA01-34-548. Fits all 1990-05 Miatas. Sold individually.