The best places right now to buy magnesium oil are online. You can get a half gallon (64 oz) of Ancient Minerals magnesium oil from Health Products Distributors Inc. (IntegratedHealth.com) for less than $115, including shipping. I'm partial to that company because I've had good experiences with them and they have good customer service. However, there are several other providers of Ancient Minerals magnesium oil, including LL's Magnetic Clay. An Internet search will yield numerous other providers.
The Ancient Minerals magnesium is considered to be the purest because it comes from a 250 million-year-old seabed that formed at the equator, but now lies more than a mile deep in the area of the north sea. This seabed is known to geologists as the Zechstein seabed. The term "Zechstein" also refers to the geologic period when the actual sea existed. The magensium from this seabed is extremely pure because the ancient sea did not contain much pollution, for example, heavy metal pollution. Other magnesium oils typically are derived from seawater. While some of these magnesium oils are highly purified, they still exhibit higher levels of pollutants than magnesium oil derived from the ancient Zechstein seabed. These magnesium oils also tend to irritate skin far more than ancient seabed minerals.
Ancient Minerals magnesium oil is the only magnesium oil recommended for oral usage by Dr. Mark Sircus. He bases his recommendation on the purity of this oil. I know many individuals who experience good results taking one teaspoon per day.
Other people use oral magnesium chloride or other oral magnesium supplements (e.g., Krebs cycle bionutrients) like Myo-Mag, and also transdermally apply magnesium oil. This can be a good idea, especially in the beginning of usage when there may be a greater deficiency of magnesium. The reason is that while transdermal magnesium is far better absorbed than orally-dosed magnesium, it can be a good idea to get several forms for maximum uptake into your body.
After your magnesium levels stabilize, then you have more choice of whether to use transdermal magnesium or oral magnesium, especially if you take care not to fall into deficiency (since oral magnesium does not rapidly reverse a significant deficiency). It is transdermally apply magnesium oil regularly, or at least keep some around because it can quickly reverse deficiency unlike any other form of magnesium, or even orally-consumed magnesium chloride. One of the reasons orally-consumed magensium (of any kind) is less effective is because the body tends to eliminate it quickly (e.g., via the bowels), whereas transdermally applied magnesium is absorbed directly into muscles and other tissues, and has much less of this effect. Another advantage to transdermal application is that the body seems to know how much to absorb, and it appears to adjust its rate of absorption depending upon its immediate needs.
I have a friend who was diagnosed with severe osteoporosis in her legs, and she was told she needed immediate surgery. Instead, she rubbed magnesium oil on her legs for a week, and she successfully reversed the osteoporosis. The doctors were surprised to say the least. She is now continuing her use of the oil so that she can maximize its benefit. She had a severe magnesium deficiency. This kind of deficiency is increasingly common in our society.
You can also use magnesium chloride flakes or gels. You can obtain Ancient Minerals Zechstein magnesium in flakes and gel form. The flakes have a large cost advantage over magnesium oil. They work well in baths, but the oil is still better for maximal uptake. The gel is excellent for skin conditions, but tends not to be as rapidly absorbed as the oil or the flakes. Whatever form you choose, you can know that you are using an effective modality that can help support your health in myriad ways. It may even save your life or the life of someone you care about.
If your kidneys are working well, then you likely will have no problem with taking magnesium chloride orally (in small quanities) and transdermally via direct application to skin. Or for that matter, taking several different forms of oral magnesium. Just make sure you get enough calcium relative to magnesium (e.g., anywhere from 4:1 or 1:1 depending on your needs) and preferably a full-spectrum mineral (or vitamin-mineral) supplement. Also note that Ancient Minerals includes many trace minerals from ancient seawater besides magnesium, so this is an added benefit.