Secondhand shops are great, but if you want to find unique items, shop at stores that are near design, fashion and art schools. The students there are likely to sell or donate their hardly-used wares at those stores closest to class. If you live in a major city, there are probably a good number of design or art schools nearby. Search online using terms like "design school", "fashion institute" and "art academy" to find specific schools and then look for stores in the same area.
Of course, people tend to sell clothes after the season is over, so you may not find spring clothing in the spring, or winter clothing when it's getting cold, but you will still have better selection at good prices for next year when the weather changes.
Thrifting Tips for Accessories with Kaileemckenzie [Refinery20 (YouTube)]