One of the great ways to purchase games for Ubuntu is with the Humble Bundle. This is a "Pay What You Want" setup. YOU name the price for the pack of games. Just got a bonus check? Pay $100. Just got a divorce judgement against you? Pay $10.
The beauty of all of this is the games are DRM free. Meaning it will work great on Linux, Android, Windows, and Mac.
The new Humble Bundle includes six interesting games with Beat Hazard Ultra looking pretty nice. If you pay more than the average you'll unlock two extra games.
These game bundles are simply awesome. Check it out!