I bought 2 pounds or so of magnesium chloride flakes from swanson.com for $15(probably find it many places), and this will make a ton of magnesium oil. For the magnesium oil i just add 1/3 cup of magnesium chloride flakes to 2/3 cup hot purified water -- it dissolves easy and when it cools I either add it to a misting bottle for applying magnesium oil spray(an easy way is to spray yourself 20 minutes before you shower, so you give your body time to absorb). Some mag-oil recipes say use 50/50 water to magnesium, i tried this but it slightly irritated my skin, so I made it a bit less potent and no more irritation.
You can also add the magnesium oil to a dropper bottle and if you wish to take it internally just fill up some empty capsules for cheap way to take it internally.