Hey,
If your wondering how to get around and about Adelaide, i would strongly suggest taking a bus which costs far lesser than a CAB/TAXI, the best way to plan your route is through the Metro website provided below.
Purchasing tickets can be done from ANY Newsagent or otherwise mentioned stores. A sign is usually put up or asking around is the next best option.
Here is the Metro Website:
http://www.adelaidemetro.com.au
Enjoy !
Vernon