Gas Bottles

Apologies if this topic has already been covered. I have just purchased a used 2023 Auto-Trail 67 Auto Pop-Top. It has had one previous owner and was supplied without a gas bottle. I hadn't realised that Calor were phasing out certain sizes of gas bottle such as the 3.5kg. Looking at other suppliers it seems there is a shortage and some are specifying that I can only swap an empty bottle for a full one rather than take out a new contract. 

I’m certain this topic must have bee covered before however just what options are now available in this situation? Incidentally, I have a full 7kg butane available, left over from my caravan days. 

Thank you.