Hmm, perhaps they are custom spun/dyed by the person who is selling the handknit items? Maybe send an email to the seller or the site to ask what kind of yarn it is - like a brand name/fiber content, etc. You don't have to mention that you don't want to buy the handknit items, you could just mention that you've been browsing the site and came across the yarn and loved it and wanted more info... perhaps you have an allergy to certain fibers and need to know the content? If they could give you a brand or something, then it would make your search a heckuva lot easier!
Good luck in your quest! <3