I was clicking through some links in my new gooooogle sidebar and uncovered this gem of information architecture: Yarndex, an online encyclopedia of yarn searchable by color, manufacturer, and weight. Every page has comments available and for wherever there are places that the yarn can be ordered there's a link.
Finding the right yarn is at least as hard as finding the right wine - see peterme's essay on shopping for wine online for a good illustration of the problems of describing something complex so that you can make some sense of it without overwhelming the reader with detail. And maybe the lesson of behavioral economics is that more choices leads to less satisfaction. I don't think you'd be happy with a store that carried every color and every weight of every yarn ever made, how would you find anything good?
From laceweight to super bulky, they have something there, though there are no entries I could see for homespun from the neighborhood's leading vendor.