When I visited Prague in 1991 and 1993 I noticed that *all* the bureaux de change were offering riff-off rates, so I went to the Bank Austria

instead where the rates for buying and selling were more or less in line with West European levels.
Btw, all the former Eastern Bloc countries have a bad reputation for ripping off tourists and, alas, the Czech Republic is no exception. As my landlady in Prague said, 'Lots of people can't see the difference between free entreprise and crime.'
In both Bulgaria and Romania one finds commission and non-commission bureaux de change within a few yards of one another. They tend to offer very similar rates of exchange but the commission places charge 8% - yes, jawhohl - 8% commission! The locals all use the non-comission places.