It's a good (BAD) time for you to inquire, cuz Cube accessory availability has been on the decline for the past year (Nintendo DS aka Dead System).
Mad Catz has made MANY variations of S-Video+A/V cables. Their "shielded" cables (Gamecube-only or Universal) are good quality and pretty durable, as I've been using them for several years. They tend to use clear plastic which allows you to see the wire-coil or braided shielding around the wires. These are the kind you should look for. I think they stopped making them, tho. The kinds I only see now have thin, flimsy wiring and no shielding -- avoid these. All models are typically US$10, and much lower if you find them for sale/clearance.
Pelican -- avoid at all costs. Generally thin wiring, no shielding, broken on me on many occasions (between my N64 and GC), and one set I bought for a friend ended up cutting the resolution of the image in-half (major huge pixels), which i didn't think was possible.
When looking at other 3rd party cables, pay attention to the quality of the construction. Thin wires along any portion of the cables are not a good sign. Shielding is a plus.
The best 3rd party cables are made by Monster, hands down. Very clean picture, little to NO noise output in the audio (when looking at GC-to-PC audio recordings). Built tough, and with Monster's quality shielding. These are what I use for my video captures. Their 6ft model is around US$20.
Nintendo's first party cables shouldn't be bothered with. They're generally more expensive than 3rd party cables (US$16 excluding shipping), and there's nothing special about them -- no shielding, same wimpy wiring as the default A/V cables, and they're S-video ONLY which cuts out the yellow/composite video option, whereas the majority of 3rd party cables include both. Seriously, S+A/V cables should've been included with the console from the get-go. Consider Nintendo's cables a last resort.