It depends on the shoe. I've danced for 15+ yrs and for plain ballet shoes, you don't really have to worry about them being professionally fitted. The ballet shoe will mold to your foot. I would recommend canvas shoes over leather though, they're more snug and really form well to your foot, I would never go back to the other kinds. If you're talking about pointe shoes, then you really need to be fitted for those. If you're ordering online, they will typically have fitting guides for you. I think my current shoes are a 7 and I normally wear an 8.5 in street shoes. Better to get on the smaller side though because they will stretch with your feet.