Since you're asking where you can buy the OS I'm assuming you're coming at it from a new perspective - in which case a distribution like SUSE, Ubuntu or Fedora would be a good choice. The main difference from Ubuntu to Fedora is that Fedora has a much stricter view on what goes in its distribution's package management - it really doesn't like closed source things, and while that's a great philosophy to help drive open source development, installing things like flash can be rather frustrating as a result. Ubuntu takes a lax-er attitude which is more convenient for most people.
That said, if you are completely new and want to buy a boxed retail copy of SUSE I wouldn't rule it out, despite what a lot of people would say. You get some printed, clear manuals, DVDs with lots of software on and the money you spend will be driven back in to help continue open source development. You CAN get it for free online the same as with any other linux distro (and most people do) but if you want a nice boxed copy, that'd be the route I'd recommend taking.