In our area, Walmart has washing soda and borax in stock more consistently than others.

FYI "washing soda" = sodium carbonate = 'pH Up' aka 'pH increaser' aka pH+', etc. Contrary to some pool store villains, it's not almost the same, it's exactly the same as what's sold as pH+ and so on.

And it too, will keep indefinitely, though it will turn into white lumps if it gets damp.