Apple Store online is where you buy the redemption code that works in the App Store.
• Lion OS X 10.7 - http://www.apple.com/shop/product/D6106Z/A/os-x-lion
• Mountain Lion OS X 10.8 - http://www.apple.com/shop/product/D6377Z/A/os-x-mountain-lion
Are available for purchase this way, as there isn't an App Store direct option.
You should find out and check details on which one best suits your interests
and realize you need not buy both, you only need 10.8 if your Mac can use it.
Either one or both require a purchase, and once they are bought, & the code
to redeem the purchase and download in App Store is successful, both of
these OS X systems do have addition step downloads (free) to the OS X for
the final step: a Combo file; either: Lion to 10.7.5 -or- Mountain Lion to 10.8.5.
The Combo update (requires 10.7 new system upgrade first) for Lion 10.7.5:
• Download OS X Lion Update 10.7.5 (Client Combo)
And Combo update (requires 10.8 system upgrade first) for Mountain Lion 10.8.5:
• Download OS X Mountain Lion Update v10.8.5 (Combo)
So the best route to check to see if the computer can run Mountain Lion 10.8 before
buying a redemption code; if it can, the 10.8.5 OS X is a better system than 10.7.5.
And then suitable applications or upgrades would need to be bought for the OS X.
{After either suitable update file is applied to the new OS X installed, also run
the software update (App Store) again to see what other updates may apply.}
Identification of the computer is essential to upgrade the hardware for later software
to run, & to tell the limits of the computer to handle newer most recent software in it.
Details on How to Identify the Computer & the last supported OS X with these sites:
• How to identify MacBook Pro models - Apple Support
The suggested total memory (RAM) upgrade mentioned here may vary from actual:
• MacBook Pro: How to remove or install memory - Apple Support
Apple MacBook Pro Specs (All MacBook Pro Technical Specifications - everymac.com:
http://www.everymac.com/systems/apple/macbook_pro/index-macbookpro.html
There are additional means, including "identify by serial or model number" lookup:
http://www.everymac.com/systems/apple/macbook_pro/index-macbookpro.html
or https://www.powerbookmedic.com/identify-mac-serial.php
In any event...
Good luck & happy computing!