Other folks have already said this, but some species are intermittently available. So if they aren't up at a given time or on a given day, they might be later on. Sometimes a whole bunch seem to be dumped on the market at once. I haven't noticed that arctic foxes are especially rare in the franchise market, but it's been a while since I've purchased any for a zoo. It's certainly true that some species, such as lions, nearly always seem to be for sale (and for a high price, so demand must be high), but others not so much. I suppose it a function of which species most players think are "essential" for their zoos? Also, lions are especially fertile as big cats go, and people tend to have prides with more than one female in it. So franchise zoos with lions tend to end up with more than they can house pretty quickly.
Arctic foxes are very, ah, fecund, so when someone does have them in a franchise zoo they tend to overpopulate. I often get the feeling that if I turn my back for a few minutes and focus on something else in my zoo, two foxes in a habitat are now 12 mature foxes that are fighting for lack of space (or inbreeding if one of the parents has died). It seems like there would be waves of foxes on the market, when a player who does have them ends up with a whole bunch at once. Maybe this rapid maturation rate paired with a relatively low lifespan discourages some folks from keeping them? Would be nice if there were a setting to move juveniles to storage when they are mature, though habitat storage fills up quickly too. I've noticed a sort of "boom bust" cycle in the market with fennecs and red foxes too.
The Frontier "zoo" also puts animals up from time to time. I don't know if they have an algorithm that checks for low availability or not, though that would be cool. The Frontier animals are often poor quality, but now and again a decent one comes up. At least they can be used to widen the gene pool of one's breeding colony. I've found that breeding a good quality animal with a "meh" Frontier animal can yield at least some good offspring, depending on how the "genes" line up.