Agreed. Just get a Pro controller and switch Shield from L and R to ZL and ZR, and boom! pretty much the same set up as the GameCube controller, AND if you've been playing SSB3D the face buttons shouldn't feel awkward. But for the sake of debate, nothing's better than the GameCube controller for Smash, and SSBU, atleast technically, is the best Smash since Melee.
I heard that they're getting restocked in the UK, so Nintendo's clearly aware of the demand, before the game comes out in Japan. Also, the price on Amazon's third-party retailers for the adapter went from around $160 down to $110 in two days, so this shortage does seem like it'll be, hopefully, a one time thing.
P.S. if you're planning on buying one of the new GameCube controllers, the L and R shoulder buttons don't have the same springs in them, but they still work fine, IMO. The controller's surface also feels different from the classic GameCube ones, but I can't tell if it's because I haven't put, like, 500+ hours of playtime into it, like the others. I would say, stick with the ones you have from back in the day OR, if you absolutely need a new one, order the 2008 white japanese edition off of Amazon for $5 more (it will have the crazy long 10 ft cable too).