Same place they'd get low octane I'd imagine, I'd guess they have to bring it out from a gas station just like anyone else,
I don't know everything,, you'll have to ask a lumberjack to know for sure I guess

I'm just relaying my experience
I've never changed mine from the factory tune, it runs great on 91 or better, crappy on 87,
Brison recommends 91-93 in the owners manual also,, I'm guessing they are tuned for that fuel