I have used banks & post offices (which sometimes run the national bank). Many years ago I used kiosks on the street in London, but on my last trip there I favored bank tellers & ATMs.

My best rates have typically come from ATMs, though - especially when I am in Canada. You should look into this option & whether your ATM network has a presence overseas, as well as the added fees that apply.

I think every country will vary, but many places post their rates so you can do some comparison shopping. A big problem is the closing times, which can be quite early.