Quite often, at multiples of the official exchange rate. In the 80s, exchaning dollars for zloty in Poland would return about 5-10x the rate you’d get at the bank (in other words, where the bank would give you 1000 zl for one dollar, the street would give you 5000-10000 zloty). There’s still plenty of places like this, Uzbekistan being the last one I visited, where the street rate was around three times the bank rate. Buying dollars the other way around was pretty much impossible, although I can’t remember exactly why, else obviously it’d be pretty easy to take advantage of the disparity.