A concern many dance teachers share is the learn-ballet-online concept. Learning ballet from scratch without a teacher is simply not a realistic goal. In fact, the chance of avoiding a dance injury is almost nil. If you cannot get into a ballet class, there are many styles of full body workouts that fulfill the healthy requirements for building muscle, flexibility, and rejuvenating yourself.
I've looked at many ballet workouts that you can rent or buy on DVD. They are suitable for retired professionals or advanced students who know what they are doing. Although some come with ballet instruction for the basics like plies and tendus, it is a basically look and copy format. I do not see how anyone can get any good out of this.
Most people want to feel the elegance and grace of movement and that is why they choose ballet. Yet there is no such experience to be had struggling along with a routine that you don't understand.
The basic Pilates DVDs could be learned if a viewer studies every detail carefully. Pilates is wonderful for the elongating movements of the limbs, and careful strengthening of the core muscles. However, I still recommend classes to get you started, and more classes when you feel you can do a more advanced routine. When is that? Not when you get a little mentally bored with your routine, but when your body can do it easily and you need more challenge to your muscles.
At that point, it is best to go to a basic class, and tell the teacher you feel you are ready to do more, but that you want to make sure you are doing everything right before you move on.
The Classical Stretch DVDs are excellent for getting a good explanation of the exercises, and also getting the basic version suggested that you can do if you are not ready to do what you are looking at. The routines are developed with a balletic style and are also elongating. They combine ballet and Pilates and also draw from Tai Chi a little.
There is even a Classical Stretch DVD to help you improve your golf swing!
The how-to-learn-at-home scenario is a very individual thing. Depending on your background with exercise and ballet training, you can certainly find something. (if you have health issues or previous injuries, check with your health practitioner). Look at the reviews on the products you like, and start carefully. That will do the best for your muscles and help you avoid injuries.
You can look at many workout DVDs here with descriptions and reviews.