A quick google shows a chandlers that says it sells Gaz,. Also worth trying would be a hardware store (Browns), garage and Trek shop. Incidentally I've found builders merchants often have gas but there are none actually in Tobermory.
Cheers for that. I'd already tried Trek, who were confident they don't sell gas. I gave Brown's a call and they thought they "probably" don't, and Seafare chandlers were fairly sure that they did. I'm not sure of the reason for the outbreak of indecisiveness up there, but I'll probably just take enough gas for the whole of my trip just in case.