Some good advice given above and the only thing I'd like to add is that if you do think about buying your own cylinder, buy two.
The reason is that there are some places where getting a fill after your first dive is either difficult or impossible, Kimmeridge springs to mind.
Or ... sometimes when going out on a boat, the boat will not return to port between dives.
Or ... if air is available there might not be enough time to get a fill, e.g. you might find that your boat is leaving in an hour and the air station is full of cylinders to be filled and it's going to take an hour and a half to get yours filled.
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