With a cya of 45, you should shock up to 15ppm. Test a couple times a day and each time, bring your chlorine level back up to 15. After your pool is clear and you are no longer getting a CC reading, your chlorine level should be maintained between 3-6 all the time. If you drop below 3, you risk an algae bloom.

I'm going to let someone else answer your question about where to order refills. As far as calcium, what type of pool do you have? If it is a vinyl pool, you don't need calcium.