Saltwater hardness??? I don't think that's the term used. Is it calcium and alkalinity you're looking for? Or is it pH?
Red Sea's test kits for calcium are suppose to be easier to use and read. I think they're all a pain to deal with...LOL. Persoanlly, I've become accustomed to Salifert's Calcium test kit and Aquarium Pharmeceutical's alkalinity test kit. pH, I use Aquarium System's 'FasTest' test kits from their master marine kit (which also includes ammona, nitrite, and nitrate tests).
These are popular products carried by most stores that have aquarium supplies. Just do a Google search on the products to find where you can get them online.