I have been very happy with my switch to the SmartPak Smart Flex joint supplements. I didn't take my switch of products lightly as my one mare was doing well on her old supplement, but it was difficult to get. The SmartFlex has all the same ingredients, and then some.
I've been using Horsetech TriSport at the recommendation of another cother. If you write the company they will send you a free sample, which is actually more like a 15 day supply! (2 lb bag). Jury is still out on its effectiveness, I haven't been using it that long. But it is available in larger sizes, as well as in Smartpaks.