If you're looking for a straight Xbox - S Video cable, then I've got a Gamester Hi Def lead that might sort you right out...The output from the console goes into a block that then outputs (deep breath) S-vid, composite, component, stereo phonos and optical audio. The stereo phonos and either s-vid or composite can then go into a scart adapter. If you're PJ'd up, can't you just use component? Possibly a silly question, but I have to ask.
If you're interested in the lead, PM me, as I don't need it anymore (RGB scart now in place)