You could use trichlor, but the CYA rise will be slow, and your pH may tend to fall, which you don't need. I would just buy the stabilizer by itself--you can get it in the spa section at WalMart in 4 # jars. It's labeled "conditioner" or "balancer", I can't remember which--but look at the active ingredient--you're looking for cyanuric or isocyanuric acid. You can add it through the skimmer but not backwash for several days, or add it in a sock or old knee hi and hang it in front of the return.

If you do opt to use trichlor instead, go ahead and pick up a box of Borax too, and keep an eye on your pH.

Janet