I have used the transdermal magnesium. It definitely gets into the body. IK made the mistake of rubbing it on, all over, after a shower, and developed loose stools from it. Now i only apply in smaller amounts.
It is important to buy the purest grade you can get rather than buying the best buy (price) you can get. The less expensive brands have more impurities and some people have reported burning sensation or intense itching from the stuff with the impurities in it. I recommend getting the Ancient Minerals brand.
Possible websites include
I am sure there are others that sell it too.