Welcome to our forum.
I typically order from:http://www.spadepot.comorhttp://www.rhtubs.comBoth have a good selection and pretty good prices.Ken
Be sure it's dichlor you're getting and not tricholor, which I believe is what those pucks often are. Using trichlor isn't recommended, and can void warranty coverage on some spas.
Best place to get your chemicals is one of your local spa dealers. The internet if very convient but unfortunatly they don't give you much help on your problems. They are strickly there to sell you chemicals and not educate you about them. Go to a local spa dealer and get THEIR chemicals and let them educate you, that is their job!