Stretch Band Exercises
Stretch band exercises are simple and easy to do, and best of all, there are lots of videos online for various stretch band workout routines for dancers to exercise their ankles. These routines also often include additional steps to the ankle excercises that aid in toe strength and pointing.
On Your Own
Another thing you can do is (and the best part about this is that you can do it anywhere, anytime) spell out the alphabet with your feet. Work you way from A-Z with each ankle, repeating as you see fit. You could also spell out words or phrases. By doing this, you will excercise your ankles in all directions, and you can do it at any point; watching TV, sitting in class, anywhere!
Bringing the Studio Home
You can also just take time to repetitively bend your ankles. For instance, take time before bed or while you brush your teeth to do plies and releves as well as simple elevese to add to your ankle strength.
The most important part of building your ankle strength is dedication. You have to make exercising your ankles a part of your daily routine. Keep it up and you'll have ankles of steel in no time!