Yep, Lowes or HD for Driveway heat. Bag or jugs..... If you supplement only your ALK, I believe it will drive you Ca lower, hence the 2 in two part.
Randy, and our own beloved Doc Roy probably highly recommend dosing the same amount of both parts when you do dose. You just have to figure out what your usage rate is on a daily basis.
RHF also has a good starting dose amount on his recipe thread in the chemistry forum, based on what is in your tank.
Unless I am mistaken, the Dow Flake is for Ca, not ALK. For Alk they use baking soda as a supplement.