Digital video output and S-video output are two entirely different things.
The only place you can by the Component Video cable (digital) is from the Nintendo Store online I believe. You're only going to need this if you have a progressive scan TV (pretty much only the newer, more expensive TVs).
As for S-Video. There are tons of 3rd party made ones as well as Nintendo branded one which isn't necessarily any better. Theres a Mad Catz, Pelican, Target brand, Monster brand, etc. If you're TV has S-Video input and does not have component, then you want this guy. One end will fit into the GC, while the other end will fit into the S-Video plug on TV.