Im lucky enough to know a bloke who supplies gas to pubs/clubs. I got a full size bottle of c02 for £15, no rentals and no questions asked. When it runs out i just ring him and he gives me another full one and takes the empty one back. Again for just £15.
Ive had my current bottle almost a year, not empty yet but getting down. Like i said im lucky because before him i couldnt get gas without having to pay a fortune.