Answer:
Actionscript 3.0 is not exactly something that you 'get'. Actionscript 3.0 is a programming language that is tied in with Adobe's Flash Player; it allows you to write code that, when used with the Flash IDE (Integrated Development Environment), helps you to create rich web applications, such as the games that you see on Miniclip.com and so forth.
If you want to 'get' AS3, I think what you're looking for is actually the Flash IDE or another way to make Flash files. You can purchase the Flash IDE, which creates flash applications (SWF's), from Adobe, here:
http://store1.adobe.com/cfusion/store/html/index.cfm?event=displayProduct&categoryOID=1882569&store=OLS-US
Note that the official Flash IDE, created by Adobe, is the easiest one to start learning with, but it is also the most expensive one, weighing in at US$700.
If you'd like to learn and publish JUST Actionscript, and find another way to draw, animate, etc (AS3 can do all of these, but not as easily as if you have the IDE to help you), you can download programs such as FlashDevelop (flashdevelop.org) for free.