It's not just limited to amiibo though, is it? The Gamecube controller adapter is extinct. Same goes for the Pro controller. Haven't seen that in a shop since near launch and they were scarce back then. They're pretty bad when it comes to stocking accessories in general. Outside of Wii remotes and to a lesser extent nunchuks it's very difficult to acquire peripherals.
Like Murr was saying his gamepad charger was chewed by his cat and can't buy an official replacement. Mine knackered too, so I've 1 charger between 2 Wii U's. That reminds me of another rarity, the official high capacity battery for the gamepad. Never seen it in the shops. Only a tiny amount of them available from online retailers.