The top up vouchers you can buy everywhere. Filling stations, supermarkets, Trafiken (Tobacco and newspaper shops) a.s.o., PAGRO paper stores and many more sell them.
Or you use
www.onlineaufladen.at (Credit Card, Giropay and others)
Just check if the SIM is still alive when you enter into austrian network coverage. In general it is not possible to find out details about the SIM's condition from abroad if the number is not registered on the A1 website because the bfree prepaid data cards do not roam. Top up from abroad is only possible on the A1 website (for instance you enter the code you bought on onlineaufladen.at) as far as I know.
The balance is the same price as a new SIM with balance (20€ or some cents less), so don't worry.