How can i get a cast for my ankle?

Visit a: Foot doctor or orthopedist......assuming its needed they will apply one.

Ankle support: Ankle braces can be acquired over the counter at the local pharmacy. Unfortunately, the really good ones are prescription. An actual fiberglass or plaster cast for a fracture treatment is usually done in a doctor's office and changed on a biweekly or so basis (more often if the patient has diabetes so that skin changes can be observed).