Is there a particular store someone can recommend going to to get this? Does it have to be obtained from a pool supply store?

pool supply or some hardware stores might have it. Just make sure its 100% sodium carbonate. Ive seen plenty of pH up that contained other things.

Alternatively, you can just buy some baking soda (sodium bicarbonate... make sure its 100% sodium bicarb), and you can make sodium carbonate by just putting it in the oven on max for a couple of hours, here's the explanation in the wiki