US retailer GameStop has recently updated its online site stating that the GameCube Controller Adapter for Super Smash Bros Wii U is now back in stock. The adapters for the popular brawler have suffered a considerable shortage in both the US and European gaming markets since the game’s release last month. According to its online site, GameStop will not be shipping orders for the adapters until next week, December 15. Currently priced at $19.99, if you’ve been looking to purchase one make sure you do so quickly at the retailer’s website, here.
http://mynintendonews.com/2014/12/08/gamecube-adapter-for-super-smash-bros-wii-u-back-in-stock-at-gamestop/
I think it's safe to assume that the Adapters aren't discontinued at this point