Aardvark's on Main Street has been selling vintage American clothing since 1971 and stocks an unusually thorough selection of denim, western shirts, military surplus, and outerwear from the 1950s through the 1990s. The organisation is by item type and decade, and the turnover is high enough that the stock changes regularly.
It sits at the lower end of Main Street near the Venice border, away from the tourist circuit, and the prices reflect a shop that sells to people who actually want the clothes rather than to people buying nostalgia. If you are looking for vintage Levi's, Pendleton shirts, or Wrangler jackets, this is one of the better places in the LA area to find them.