I use Fox farm ferts, and they contain magnesium, but there are other complete fertilizers that contain it also. However, as a precaution, I use one tablespoon of dolomite lime per gallon of soil prior to planting.
I only had a magnesium problem once, and that was when I tried going all organic. It might be a bit more difficult supplying the magnesium when going organic; I don't know. I'll try organic again another time. The only time I went all organic I didn't know about dolomite lime. It might have worked out better had I used the lime.
By the way, are you doing an organic or inorganic grow?