There are a few auction houses who specialise in all things vintage and there are also other auction houses who have a textile sale every so often. The scarves sold in auction houses tend to be rare and very valuable from private collections and the condition is usually near perfect. Also scarves can come under the banner of fine art prints and are sold via this avenue in auction houses rather than textiles. In auction houses you may find that scarves are sold in bulk and within the bulk is one scarf in particular you may want. Remember to account for tax and buyers premium.
Here is a list of some auction houses;
Kerry Taylor Auctions http://www.kerrytaylorauctions.com
William Doyle http://www.doylenewyork.com