When faced with the problem of how to convert your currency to soles, you may be paranoid about finding the best conversion rate. No one wants to get ripped off, right?
If you're trying to save on those little decimal points, then you want to avoid taking soles out of an ATM or out of your account. Instead, take out your money in your own currency and then go to another changer to convert it to soles. When you take out soles from the bank and let the bank do the conversion rate for you, they often charge a fee for their services and they normally don't have the best conversion rate.
Once you've taken out your money, look for the small money changing stores that line a specific street in the city center. In Cusco, most of these money changers are located at the top of Av. Sol near Plaza de Armas. Every Peruvian city has its own specific location and everyone will be able to tell you where to change money. There you can find a much better exchange rate. Take note that your changer will stamp your soles with their special mark. If you need to return a bill for any reason, you can use this mark to prove they issued the bill. If a bill is too old, ripped and worn, many stores and vendors will not accept it. You can either return it to the converter, or find the closest bank. Both will exchange the damaged bill for a good one free of any charge.
Supposedly the money changers who stand in the street with huge wads of bills crying their services have the best exchange rate. The problem with using them, of course, are the thieves who are watching the every move of people who use their services. If you do use a street money changer, be very careful about where you are, who else is there, and where you put away your soles.