Instructions
Know what you are looking for. Vintage Levi's jeans from the 1950s have a Levi's label that is printed in all capital letters. The jeans were labeled this way until 1971. Levi's jeans manufactured after 1971 have the Levi's label with a lowercase "e" in Levi's. When you are looking for authentic vintage Levi's, you must look for the big "E."
Shop at thrift stores in your local area. Most thrift stores carry used and donated clothing and you never know what you will find. Many of these type of stores put new merchandise out on Saturday mornings. Check with the management of the store to find out when new clothes are placed on the floor. Visit the store once a week whenever new clothes are put out for the best chance at finding vintage Levi's.
Shop on eBay. Use the search term "vintage Levi's" to find what you're looking for. Make sure that the seller posts a photo of the label so that you can check to see if the jeans were made before 1971. If there is no photo of the label and the seller refuses to list one, the jeans are probably not authentic.
Visit local garage sales on Saturday mornings. The earlier you get to these sales, the better your chances are of finding vintage clothing items. You might have to visit several garage sales.