Kate, magnesium is readily available at the drug store and grocery store. Plain old Epsom Salts. Publix sells it in a package like a 1/2 gal. milk carton for like $3. I have Dr. Teal's lavender scented bath salts . . . I soak myself in that as well as the orchids.
Since it's so soluble, it gets leached away very quickly in the heavy summer rains, so it's one thing you can hardly over-use.
