I've had good experiences at thrift stores, especially local ones. While they're definitely filled with basic stuff like t-shirts, I've found plenty of cool stuff too. They're not one-stop shops for sure though, you'd wanna do a few stops at a couple to get a really diverse wardrobe. I'm also the kind of person to wander and look at other stuff in thrift stores too, I've found some cool historical stuff about my area in them.

As far as the organizations behind them go, I'd generally try to avoid chains like Goodwill or Salvation Army because they've done (and probably are still doing) some sucky stuff, if I remember right Salvation Army donates/d to anti-LGBT groups and Goodwill exploits disabled people, but I have no sources on hand so take it as you will.