I know this isn't the cheapest it can be found for, but for under $25 including shipping it's at least in stock and easy to get.
(scroll down)
http://www.tronixweb.com/shopsite_sc/store/html/gamecubeusq-z.html
A warning: their website doesn't have secure ordering so use a temporary credit card number if possible.
(scroll down)
http://www.tronixweb.com/shopsite_sc/store/html/gamecubeusq-z.html
A warning: their website doesn't have secure ordering so use a temporary credit card number if possible.