Finding a place to change money in Peru was a breeze — IF you are exchanging dollars. I should note Euros are becoming more common.
There are exchange stations everywhere, however the rates are dreadful. You can go to the banks but you will queue for ages. The best way to get cash (Peruvian Nuevo Sol) is from an ATM or in advance from your bank or some other company such as American Express or Thomas Cooke.
For those of you from the UK, I should note that pounds are not as widely exchanged in the little shops and street vendors. US dollars however are everywhere. In fact you can usually buy things using dollars. Services, hotels, tours, etc are often priced in dollars. Keep in mind that you will receive change in sols.
The problem comes in with the dollars themselves. If you do plan to pay with them or want to exchange them for sols they’d better be in MINT condition. If the bills are well worn, have even the tiniest of nicks or tears, or just aren’t crisp enough people will not accept or exchange them.
Honestly, you’re just better off using nuevo sols. I saw no advantage at all in having gotten dollars before the trip, in fact since some of the dollars we had weren’t perfect, it became a problem.
Our advice, bring very little if any foreign currency with you. Just get sols from a bank or ATM on arrival.