Dowflake is easy, I've purchased it from a Pinch-a-Penny pool supply in Plantation. They carried other brands, but I found one that had dowflake specifically.
Magnesium is a little harder to come by. We did a group buy about a year ago for 3-400 lbs. worth, but I don't think that vendor is selling anything less than a full truckload anymore. You could always try twopartsolution.com, but you'll probably pay a lot more than the $50 for a 50# bag we got last time.