When we flew out there in June 2009, William Creek had an avgas bowser. We had previously rung Wrights Air to verify this before we departed. Both Tibooburra and Marree had drums only at that time, and only by pre-order as well.
In the end we purchased fuel from bowsers at Broken Hill, William Creek, Oodnadatta, Birdsville and White Cliffs. We didn't use any drum fuel on that trip.
Rattly