There is a Hawkshead store. There are also a few campsite close by if get didn't stock what you need.

Pretty sure you will be able to get what you need in Pitlochry but if your are passing Blair Atholl then should be able to do a gas resupply at Bridge of Tilt campsite (that's where I got mine last time)

There was a good little hardware store in Pitlochry that according to Google Streetview was still there last year. They are almost certainly likely to sell gas. https://tinyurl.com/y9hd4cnl

The shop at Faskally Caravan site is a decent size and sells camping supplies.

Hawkshead's details are:
Hawkshead Pitlochry, 132 Atholl Road
Pitlochry,PH16 5AB

017964 70491
pictlochry@regatta.com
Click here for their website

It's their flagship store.