I agree, call ahead.
I had difficulty sourcing a correct canister for a jet boil, for last years TGO.
After phoning around several outdoor shops, I located one at sports direct Aberdeen. I explained I was traveling down from Shetland the following week and they kindly agreed to hold it for me. They wouldn't take payment for it, until I picked it up.
Excellent service!

Hi Marc,
You can also try Nordic Outdoors, just off the Rose Street. They were selling gas last year when all the other stores ran out.

A bit further (the other side of The Meadows) but easily walkable is Alpkit, they had plenty of gas canisters last time I visited.

If you need it urgently and no lack, give me a shout by Friday night and I can give you one.