Ebay is a great resource to begin your search; I have purchased a gorgeous Chloe coat, a vintage Chanel purse, and countless other treasures BUT you must be prepared to do your homework.
1. Find out if the seller is from the U.S or overseas.
2. Make sure that the product states that it is authentic in the description.
3. Only buy from a seller that has good feedback.
4. Ask for receipts or where they originally purchased the item.
5. Be able to identify fakes; this is a great website to help http://mypoupette.com/
The local thrift shop or Goodwill are also excellent choices to find hidden treasures. Here are some pointers during the hunt:
1. Make sure the item is made of a good quality fabric.
2. Always try it on because you never know if it was altered or what type of sizing (U.S, French, English, Italian).
3. Look at the linings to make sure they are intact.
4. Just because it's designer doesn't mean it's worth buying, make sure you like it and it fits properly.
5. Look at where the item was manufactured, this will give you some knowledge to the quality of the craftmanship.
Here are some of my favorite vintage stores in the Seattle area