You can download it free from any one
of many locations.Go to Google. Type in "linux"

or you can find packaged distributions in software
stores.

Often, when you buy a good book at a book store,
it will also have the install CD.

It takes quite a while to download unless you
have a very fast connection.

Even if you pay for a package, the software is free,
inthe sense that it is legal for you to make as many copies
as you want, and give them away, sell them etc.