Mac OS X 10.5 "Leopard" is $129, available from retail outlets the world over. Your best bet would be your nearest Apple Store, but the big online retailers (amazon.com, buy.com, etc.) also have it. Make sure your computer is up-to-spec with the system requirements:
http://www.apple.com/macosx/techspecs/
You have three choices of installation:
1) Archive & Install -- preserves your applications and user accounts and installs Leopard.
2) Upgrade Install -- preserves all your applications, user accounts, settings, etc. and upgrades you to Leopard.
3) Clean Install -- erases everything on your hard drive and installs a fresh copy of Leopard. Doesn't preserve anything, so make sure your files are backed up.