@totty My wife has PF .....and we have all three oxygen sources. There was never any resistance by our supplier to give us what we asked for.....they were extremely cooperative.
Are you going thru Medicare on the billings? Or private insurance, or paying out of pocket?
It seems to me that this supplier of yours is making up it's own rules. There's no reason for them to deny you the canisters, as long as your doctor prescribed. and they get paid, one way or an other. It just doesn't make sense.
Perhaps try speaking to someone higher in authority at your supplier. It could be that whomever is telling you this....is misinterpreting the rules. Is there another supplier available to you?
Finally, consider reaching out to the Pulmonary Fibrosis Foundation, for help. (PFF Help Center (844) 825-5733) or http://www.pulmonaryfibrosis.org .....they have some outstanding resources to help you.