You can get your refund in several places in Austria.
Remember: No customs stamp = no refund

Airports (get a customs stamp before you check in):
Vienna
Innsbruck
Salzburg

Travelling by train:
If travelling by train from Austria there should be a customs officer on the train before crossing the borders. Simply ask for a customs verification stamp.

Travelling by car:

Borders:
St. Margrethen (Switzerland)
AU (Switzerland)
Dornbirn
Diepoldsau (Switzerland)
Nendeln (Liechtenstein)