If you still haven’t been able to get your hands on the GameCube adapter for the Wii U, you may have to resort to shopping with GameStop and buying it in a bundle. The company just updated their website with the adapter, which is not available individually as it is still sold out. However, GameStop is more than willing to sell you an adapter if you’re willing to buy a GameCube controller alongside it. Not only do they have bundles with just a single controller, but they also have bundles with up to four controllers.
Of course, these bundles aren’t made by Nintendo or anything and GameStop is perfectly capable of selling the adapter to you individually, but how else would they make money if they didn’t upsell you a few controllers, too? For the more sensible, keep an eye on the GameCube controller on the NowInStock tool to find one at the actual MSRP instead of having to buy extras you might not need.