To purchases the official version, go to www.robotc.net
and select Lego Mindstorms on the left hand side of the page. The next page will have the button Purchass on the left hand side and that will take you to the different packages available.
You can buy an Online License, were you pay online and are given a license code to activate Robot C from any downloaded version.
You can also buy a Robot C CD / license package. You will be mailed a CD with a license to activate it. I don't know what version the CD will be.