Where do I go to buy/download boot camp?

Please find your Mac OS X version and read the appropriate manuals and guides carefully before installing.

http://www.apple.com/support/bootcamp/

http://support.apple.com/kb/HT4818

http://support.apple.com/manuals/#macos

You have it already in your Utilities folder. It is called Boot Camp Assistant.

and it works a treat, If you're feeling geeky you can do it all manually using Disk Utility and the windows installer disk, just be sure to install windows on the correct partition! (backup)

In addition to the excellent advice from BobTheFisherman and dalstott I strongly advise you to Backup your OSX installation before you start altering/adding partitions.