This is it... Pro CMC Cal + Mag 64 oz, Pro CMC From Adams Horse Supplies
Most tack shops with a good health & medical section will have it. BTW, if you DO have an ongoing problem, I would highly recommend Succeed. It's expensive, but works like a dream! My vet recommended it and it worked instantly. I used the Succeed syringes for a month, then put him on the daily powder supplement. I haven't seen any runny poop (or pre-poop squirts as I referred to them) in months.
I was spending more on baby wipes to clean his hiney, and blanket cleanings to clean the dried poop off the back of his blankets than it cost to keep him on the Succeed and a bottle of Pro-CMC in the cabinet.