Check out Ben's Best Guess Chlorine chart, this will give you an idea about why you need CYA and where you should have it. The easiest way to add it would be to make a trip to the pool store and buy some, normally pool stores call it stabilizer. I bought some when I refilled mine and it was around $36 a gallon if I remember correctly. That was the liquid version, though and that's all I've ever used.

Be sure to stick with either bleach or liquid chlorine, as using any stabilized chlorine will just raise your CYA levels and require you to keep your CL higher to be effective.