Insurance
An insurance company can get it for you but it has been my experience that it can be gotten less expensively from massage associations. There is also a massage magazine that was offering it at a really good price but with less coverage.
Each massage therapist would have to be covered so you may have to require them to get their own insurance unless you go the insurance company route, then all present and future employees can be automatically covered.
In any case, make sure your insurance covers professional as well as general liability. Don’t be hasty; shop around and get the best price.