I'm with Air Liquide. However they all pull your pants down on bottle rental. It's a bloody rort.
There was a crowd in NSW I made some enquiries with a while back that let you buy the bottle outright. They don't have an outlet in Vic yet.
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The other option you may want to consider is disposable bottles. My bottle rental is about $150 per year and a refill is about the same. Disposable bottles run about $60 each so you can do the sums on which works out cheaper.
Cheers