I need me one too. That Gamecube controller cord is short as hell. A few months ago they were all over (that and memory cards being the only Gamecube accessory I've seen) but now they seem to have disappeared.
You can order the silver Wavebirds from Play-Asia for $39.90 +$8.80 shipping. Otherwise, you'll either have to try your luck at a used shop or on ebay.
When I got my gamecube a few months ago I just got an extension cable from gamestop like monkeydeew said. They only cost $2, and the one I have works fine.