I doubt Leslie's has any R0871, though you could certainly check. If it's much of a drive, I'd call first though.

But, you are right about the R0871 -- I had overlooked the fact that we need to warn people that the high chlorine & cc levels experienced during ammonia clean burn through the R0871 in a hurry. It's a bit of a problem, since there's no real fast way to get more. I just looked on Amazon, and this is probably your best bet:
Taylor FAS-DPD Titrating (Chlorine) 16 oz R-0871-E
You can refill your R0871 bottle, by popping the tip out (press your thumb on the side of the tip, while holding the bottle, and press), and then refilling. You might want to try doing so BEFORE you order the big refill, since if you happen to ruin the bottle, you won't be able to use the refill. Use CLEAN hands -- you don't want to contaminate the tip or the solution.

Also, if you'll store the pint bottle in the back of your fridge, where it's dark and cool, it should last several season.

Regarding your pool, I'd recommend going ahead and adding a 25 gallon dose of chlorine, at about 7 or 8 PM, so it has the maximum time work before the sun hits it. I'd also recommend putting in a couple of boxes of borax, first, to raise the pH a bit more (you reported 7.6).

You can use an OTO kit to monitor it. After the initial dose, it will probably give a dark orange brown results (~70 ppm). You do NOT want to swim in this till it drops to dark yellow, unless you begin adding stabilizer first. It won't hurt YOU, but 30 ppm of free chlorine, with no stabilizer, is a swimsuit killer! AND, 20 ppm of COMBINED chlorine WILL give you a skin rash, etc.