Ive found a couple of used bikes on local craigslists in different regions (both found by friends and referred to me) , and did look at used fatbikes on something called Pinkbike, which seems to be more of a mountain bike thing in the States, and then also on another local thing called Kijiji (which might only be Canadian).

I reckon using the right keywords is the main thing, stuff like touring, igh, rohlof, all that sort of stuff, will just increase the chances of getting relevant listings to pop up.