In reply to iain150:
The only 100% source used to be Okov home hardware stores in Montenegro (closest one in Podgorica).
https://www.okov.me/mn/proizvodi/rezer-boca-sa-punjenj-30propan-70butan-230...
I seriously doubt Theth would have any EN417 canisters – where would they buy them and why? Most people eat in guesthouses. Importing them from Podgorica or even further from Tirana? Unlikely, and I haven't seen any in the few closer Albanian supermarkets I visited, though there might be some home hardware store in Shkodër that I missed.
We hiked higher alternate routes where we simply had to camp most of the time, and it was quite a chore to find out that Okov sells them (in the soldering section, even!), back then, even though I speak the language a bit. Nobody knew, as EN417 cans aren't really widely used for camping there, so even when I asked in my semi‑passable Montenegrin, everybody just pointed me to their local incompatible cans.
You might get lucky with other hikers leaving a half‑full one at a guesthouse (it's good manners).
You could buy online in Okov and have it delivered to your place of stay in Plav, but that sounds quite challenging, unless you have the language skills to arrange it all.
You could buy a local gas stove that is compatible with local canisters. Around 20 quid, available in many more places, often even in street markets.
The local stoves look something like this https://www.okov.me/mn/proizvodi/plinsko-kuvalo-sa-zastitom-od-vjetra and use those single‑use canisters https://www.okov.me/mn/proizvodi/rezervna-boca-sa-butanpropan-punjenjem-190...
Mind you, once you install the canister, it absolutely mustn't be removed til empty, as it's pierced, not screw‑in.
But don't despair yet – I had a look through Montenegrin retailers (always interested in updating my personal notes on gas canister availability in foreign places I visit) and found out that VOLI now offers normal, EN417 screw‑in canisters: https://voli.me/proizvod/3496
Though that's only on their online shop, you'd have to check the local VOLI in Plav for their in‑store availability. I suggest calling them, preferably having some prepared phrases to ask (Google Translate is your friend).
Post edited at 14:15