20 Mule Team Borax, 4# 12 oz -- that's the only size I've ever seen.

Add chlorine this PM, test, and test again in the AM.

No rush, but you will be needing at least a gallon of muriatic acid. (Lowes, Home Depot, etc.) Pick it up when it's convenient. I'd recommend you get a heavy garbage bag, and put the acid inside THAT when you carry it in your car.