Many places sell dichlor shock. I never buy it, but I think even Walmart may. Check the ingredient labels on their powdered shock. Stabilized products are types of chlorine that have chlorine and cyanuric acid (stabilizer) in them. Trichlor pucks are very common but will not add the CYA very quickly. That is why I suggested dichlor as it will add it faster. Since your chlorine is so low and by now ( 4 hours since you posted) it is probably non-existant, use bleach for now until you get some dichlor. In your pool, each cup of 6% bleach will add just under 2ppm of chlorine to the pool. Each time you test, add enough to get the cl level back up to about 6 or 7. While there is no stabilizer in there, it will disappear fast so you'll need to test and dose at least 2x per day and if you can, you may need a third time as well to make sure you keep some chlorine in there.

Another option if you don't want to use dichlor, is to just use bleach but also buy some CYA and add it separately. CYA is called several things --- stabilizer, conditioner, cyanuric or isocyanuric acid. If you decide to go this route, add about a half a pound slowly to the skimmer while the pump is running. Then, keep the pump running 24/7 for about 4 or 5 days to give it time to dissolve in the filter. This will take you up to about 30ppm of CYA. After about a week, your CYA should totally be dissolved. If your kit doesn't test for it and it probably doesn't, have a pool store check the level for you, but don't let them talk you into buying a bunch of stuff. Report the level here so we can see where you are. Once you get the CYA in there, you should also be able to go to testing and adding bleach in the evenings only.

You are right about adding muriatic acid to get the pH down. Small doses of muriatic acid (maybe about 1/4 cup) added slowly in front of a return jet. Be careful not to get it on you or on the sides of the pool. Wear gloves and glasses and be upwind of the fumes. Nasty stuff. 3 or 4 hours later, retest pH and redose if needed until you get to around 7.4-7.6 although actually anywhere 7.2-7.8 is ok.

Let us know which chlorination method you want to do --- either dichlor for now and switch to bleach later or go ahead with bleach now with CYA added separately. Both methods will work. Do the one you are most comfortable with.

Keep us posted on how things are going!