Ask your sons' dance instructor.  

If she doesn't know, here's another website, and a phone number, for Patterson's Back Bay Dance Shoppe - http://www.backbaydancewear.com and (800) 554-2340.  They are located in Boston, but they do ship, and pronto.  I know that they outfit boys, because I was at the store the other day, and it was swarming with kids; most of them students at Boston Ballet School.  

They and their parents have descended on local dancewear stores in large numbers because this is back-to-school season, which I imagine is what prompts your own urgency. (It's also why I'm going from store to store - I procrastinated until the week before classes start, and now other guys have looted the bins ahead of me.)

P.S.:  Because it is an undergarment, a dancebelt generally cannot be returned to the store if the package has been opened.  For the same reason, a dancebelt can't be tried on, unless the dancer is wearing something else underneath it.