ß isn't beta. It's the (german) sharp s. The greek beta looks like this β.

The fastest method probably is: 

- enable the greek keyboard layout permamently in System Preferences -> Keyboard -> Input Sources
- defining and enabling a free keyboard shortcut in System Preferences - Keyboard -> Shortcuts -> Input Sources -> Select next source in input menu (e.g <kbd>Shift</kbd><kbd>Alt</kbd><kbd>Spacebar</kbd>)

To type α/β, you just have to switch the keyboard layout with <kbd>Shift</kbd><kbd>Alt</kbd><kbd>Spacebar</kbd>, hit <kbd>A/B</kbd> and switch back to your common keyboard layout with <kbd>Shift</kbd><kbd>Alt</kbd><kbd>Spacebar</kbd>.