Since I began smoking a pipe in 2011, Esoterica, Germain, Peter Heinrichs, Butera tobaccos, and Balkan Sobranie were all very difficult to obtain, and still are. Over the past 6 six years, since the closing of McClelland, and then the license of Dunhill going away, and then being re-sold as Peterson; I think many pipe smokers have begun to cellar tobacco's more so than they would have in years past. Since the above-mentioned events I've noticed that brands such as Samuel Gawith, Gawith and Hoggarth, Dan Tobacco, Wessex, and Fribourg & Treyer tobaccos have been difficult to obtain as they always seem to be out of stock at many online retailers. I'm not sure if this is due to a limited release like Esoterica and Germain's blends, or if it's due to people buying these tobaccos in large quantities when they become available. Either way they always seem to be out of stock.
Within the last few years many online retailers have listed restrictions on the amount of tobacco one can purchase (i.e. Carolina Red Flake, Anthology, etc.). Again, something that I didn't see prior to the above-mentioned events. At this point I don't think there is one unicorn, but a herd of unicorns being sought after. If I'm lucky enough to stumble across one of these "unicorn" blends then I'll purchase it, but I'm not one that goes out of my way to acquire them. I don't put myself on the email list, I don't check secondary sites, etc. I have enough tobacco to last me 10+ years at this point. I occasionally acquire tobacco's still, but I don't go out of my way to hunt down any particular blends.