When I first started dosing, I was using all the DIY stuff....
the alk part: arm & hammer baking soda, pretty cheap at a bulk food store.
the ca part is Dow Flake(calcium chloride)
the mag part is epson salts(magnesium chloride),
there is a 2 part recipe in Reefkeeping magazine, here's the link
http://reefkeeping.com/issues/2006-02/rhf/index.php
the challenge with Dow Flake is that they have changed the recipe over the last few years to Dow Extra... there is a controversy that it is not as pure as the original recipe. I used it for 6 months and didn't notice much of a difference before switching to BRS calcium chloride.
You can find it at most pool supply companies everywhere for about 10-20 bucks a bag.