There's no free Mac OS. ... Well, System 7.5.3 can be downloaded for free - as well as the update to 7.5.5, but I guess you mean Mac OS X? Isn't free. Comes either with a new Mac, or for older Macs, you can buy the newest retail version.
But be sure to get the retail version. A version which came with another computer might simply not install on yours. That can be difficult to see from an eBay auction, for example.
Make sure that you aren't just buying "Upgrade CD's" for OS X, which will not install OS X, but update it (which it can do by itself anyways). This seems to be the latest scam..