I'd like to purchase a copy of Mac OS Mountain Lion, how can I do it? I'm currently on a Macbook OS 10.5.8
You can purchase Snow Leopard contact Customer Service: Contacting Apple for support and service. The price is $29.00 plus tax. You will receive physical media - DVD - by mail.
Third-party sources for Snow Leopard are:
After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store.