Randy Holmes Farley wrote an article on magnesium supplementation a while back. http://reefkeeping.com/issues/2006-07/rhf/
You can skip way down to the DIY recipes near the bottom.
You could also go to your LFS and get some Kent or other prepared magnesium product.
Yup, agway. It's tough or near impossible to find mag flake now. It's mag pellet that you will find. The Randy's article mentioned suggests reducing the volume diluted by around 20% due to the higher concentration in the pellet.
On a similar side note I recently seen "dowflake" but it wasn't the OG dowflake pure calcium chloride, it was a blend.