If you live in a major metropolitan city, I guarantee you there is at least one studio that has open classes for adult beginning ballet dancers. At the studio I dance at (Alonzo King Lines Dance Center in San Francisco) there are definitely introductory classes for adults in both ballet and modern, and I have seen all types of people and body types take class there.
That being said, ballet is a highly difficult dance discipline and you should only take it if you are interested in ballet itself. It is not a class you should take if you only want the fitness benefits because there are much quicker ways to accomplish your goals. "Ballet legs" can be accomplished by either free weight or weight machine exercises focused on the leg, with more reps (12-15 per set instead of
to get the leaner but toned look. There are some good exercises posted here, but if you don't do them with the right ballet technique (hips squared, turned out legs, etc) you won't reap the benefits.
That being said, I have danced ballet my entire life and think it is a ton of fun, so if you want to try it drop in a class. Just note that many of the biggest USA studios (Edge and Millenium in LA, Broadway Dance Center and Steps in NYC, Lines in San Francisco, etc) have misleading course names. "Beginner" labeled classes are NOT for beginners...they usually require a complete understanding of ballet terminology and technique (for example, BDC recommends you take beginning level classes if you have 2-5 years experience). The real beginner classes are usually marked as "Basic" or "Intro" levels.