CYA is cyanuric acid (or isocyanuric acid) that is usually called Stabilizer that you can get at a pool store or store that sells pool chemicals. It dissolves slowly so you need to wait at least 5 days after application to get a good reading on where you are at. Some people add it to their skimmer and let it sit in the filter to dissolve (do not backwash if you have a sand filter) and others put the CYA in a sock or nylon hose, tie a knot and hang it in the pool. Either way is up to you. You want to shoot for a level of 30 ppm. It is also in tri-chlor pucks (and di-chlor too I believe) so if you use pucks that will also raise the CYA although more slowly than if you add it separately.

Peter