We use Keg King branded gas bottles. This map is collated by them and includes distributors all over Australia. It is a good idea to call them before bringing your bottle in as things change rapidly with small businesses.
You can also just ask your local pub where they get their gas bottles filled as there is plenty of local businesses not on this map.
For SodaStream gas bottles just swap the empty one for a full one at your local Woolies, Coles, Big W, Target, IGA, Service Station etc.
For more details on swapping SodaStream bottles see this link