can only get it online with a prescription from your vet as it is prescription only. There are loads of internet pharmacies. try best pet pharmacy - that's where I get my boys stuff from.
One of my horses was diagnosed with gastric ulcers and put on GastroGard, my vet recommended that when the insurance stopped paying to swap to Equine America UGard and V-Biotic as it gives the same results. It's much much cheaper and my horse is just as comfortable on it as she was on GastroGard.