Had a look through the Trailblazer book and Coleman style gas is apparently not that easy to get on the route.
First option I can find on the route is at
https://www.campingintheforest.co.uk/scotland/loch-lomond/cashel-campsite but the Trailblazer book warns that stocks are often low/out towards the end of the season.
The other option, which depends entirely on how and when you are travelling, is Halfords. There's a store very close to Bearsden station which is a few stops down the line from Milngavie, you'll pass through it if you're getting the train from Glasgow city centre. However opening hours might not be convenient for when you're travelling, but it would be possible to place an order online and collect it there to guarantee you get it.