You are likely to be aproached within the main shopping area by groups of men offering very competitive rates of exchange. It's a pretty simple trick; they'll grab your money and run off with it.
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Rates are better in Sofia than in smaller towns and better than on the beach resorts.
If you do exchange on the street and get a better deal feel good in the fact that you have assisted the local underworld in washing some Leva into a slightly cleaner Euro, Dollar or Pound.
Stick to the bank if you need to exchange cash or better still an ATM.