Check with your bank on exchange rates. Bring some currency of your host country with you. Acquire a small amount of currency through your bank or plan to acquire some at the airport before you leave. Money changers may be closed when you arrive in your host country. You might want to buy some food, take a bus, or a taxi.
Local currency is necessary for such activities.
On the streets, particularly where tourists are present, you will encounter nondescript men offering to exchange your currency. They normally offer a better exchange rate than the official one. Whether or not their conduct is legal we do not know. Ask your tour guide, tour leader, or excavation staff about buying currency in this way. Be sure you know what the bank and hotel exchange rates are before you buy from the man on the street.