Super Smash Bros Wii U Gamecube Adapter

Where can I buy it without paying an arm?

Probably wait 3 months until Gamestop, Best Buy, and everyone else has it in stock.

Right now, you're gonna have to give your left testicle for one.

Keep an eye on the Amazon Warehouse deals thread in the video game deals section. Maybe a good used one will pop up.

Other then that, Cookie Party is right, as there are no sales at the moment (that I know of).

Just keep an eye on the video game deals section in general.

You might try browsing the trading section here on CAG. Some other member might possibly have one for sale or trade.

Import this badboy http://www.eurogamer.net/articles/2015-01-13-this-third-party-wii-u-gamecube-controller-adapter-works-with-pc-too

Check these deals out from Play Asia -

https://www.cheapassgamer.com/topic/341248-gamecube-controllers-and-third-party-adapters-at-play-asia/#entry12402452