Viton would be my choice. Its your basic go to seal material for everything petrochemical (liquid & gas).

Spaenaur (in Canada) have them in stock

PN: OR-002N for Buna-nitrile 21.70$/100
PN: 320-002 for Viton 39.92$/100

But they are the Milsig.ca of the O-ring. Like 5 to 10 times the price of Mcmastercarr.

I tought Mcmaster carr don't take no more canadian account. Is it true?

McMastercar told me they will for businesses, but for individuals its not worth the time for them to deal with documentation and customs stuff.

Looking back through this whole search for o rings, I would personally just buy some cheap lighters and cannibalize them. This is even easier if you work with several chain smokers like I do. I found got 4 lighters in a week. You should get at least 2 o rings from each, one on the fill valve, and one near the nozzle where the gas comes out. Still expensive considering it would be 50c/o ring but its an option that works if you only have a few mags that need o rings.

Overall this is far easier and cheaper in the long run that sourcing them from a company.

Or you know make friends with your dentist and ask for some of the elastics they use for braces, that also works pretty damn well.

super cheap fix
 

16 gauge silicone wiring, that's right, the noodly wiring. pull the copper wire out leaving just the sheath, use an exacto knife to cut approx 1mm of a wee wide ring from the wire and voila! I had spare wiring from previous builds laying around, wire that unkikely get used again, took 5 min and I now have dozens of orings for all my fill valve for practically nothing cost wise.

good tip thanks!

For whom it may concern, I just thought of thesse again and ordered 100 from McMaster Carr since I'm a business. We'll see what they're like when they show up. I was planning on putting them at least in my TM MWS GBBR mags.