Most Pool and Spa dealers carry it. Numerous types are available, only sodium di-chlor should be used in spas. Depending on it's application, it may or may not be very effective. Nature2 Nature2 is one of the newer sanitizing methods. It seems to work very well and is based on the same principle as an ionizer, without the electricity.
Some people love it and some people hate it.
The major complaint has to do with all the products you have to buy and the cost. This is a newer product that has made some rather amazing claims. Many people though swear by it.