They have been selling them separately in EBGames/GameStop, Best Buy, Walmart, etc. But since those were limited in-stock, it was basically first-come-first-serve. I mean, some sellers even said that Nintendo is not planning on making more of them (don't take my word for it though). If you didn't purchase one in the first few days of Smash Wii U release, you might be out of luck. Of course, there's always Amazon or eBay, but the prices that most resellers have are ridiculous (over $100) and so they're obviously trying to take advantage of the limited GC Adapter to make some extra cash.