I had never heard of magnesium citrate either until I started doing liver flushes.
Magnesium citrate is found in the laxitive section of any drug store or the pharmacy section at Walmart. It comes in liquid form and in lemon or cherry flavors. It costs under $1.50. It doesn't taste bad at all, but it is carbonated and tickles my lips, so it takes awhile for me to get it down, using a straw!