I think you can only get from a vet, I use to have it on hand in a small tube, when the horse had a flare I would give two small scoops (came with the container) soon as possible than 1 scoop 12 hours later. Than none, only for flare ups, supplement for treatment other times. A small tube cost me under $20 and lasted through five or six flare ups.
Yes dex is suppose to be a possible cause of foundering, but if your horse is low risk in the first place, I wouldn't worry about it. I know four or five horses with terrible heaves that were on it pretty much daily and never foundered. Mind you most were switched to other supplements which seemed to control it well enough to live normal lives.