Ryle is right, go for something with glucosamine, condriotin, and MSM. There's also an acid found in citrus fruit that's really beneficial, too. You may want to have a vet check her over first, just to make sure she doesn't have anything else making her stiff. The vet should also be able to help you out with what to look for in a supplement.
I use Farnam Platform for Hoover's ankles, but just check out the different kinds. You can do a powder, liquid, or pellet. Horse.com has a nice selection, as does drsfostersmith.com.
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux