Hi, and welcome to the forum!

Many of us just use WalMart, Costco, Sam's, etc--but where the best place to get it is will depend on what you need, and you won't really know that until you have good test results. However, low pH is one of the critical things, so you could go ahead and start adjusting that now (it needs to be done before shocking the pool because high chlorine levels will falsely elevate pH readings). A pH of less than 7.0 is acidic and can do some real damage to your liner. We recommend 20-Mule Team borax (laundry aisle at WalMart) to raise pH. We usually recommend it added slowly through the skimmer, breaking up any clumps. Depending on how low your pH really is, it can take quite a bit of borax to raise it, but your pH needs to be between 7.0 and 7.8.