The key to converting currencies is to use reputable sources. Usually, the most reputable sources are those with whom you already have a relationship, such as credit card companies or travel services. While other sources might offer a more favorable rate of exchange, you should be aware that when in a foreign country, you could face arrest and detention if the "exchange service" is a front for money laundering operations.
A Credit Card Company
Many credit issuers, such as American Express and Diner's Club, offer travel-related services in their overseas offices. These services often include currency exchange, converting U. S. dollars to whatever the local currency might be. An added advantage is that these exchanges can be made in traveler's checks for added security. They charge a small fee, usually less than 1 percent, but offer the protection of a receipt for the exchange, should the source of your local currency be questioned by law enforcement personnel or security forces.
Larger national banks offer currency exchange services. The rate of exchange will be the official rate, current as of the time and date of the exchange and there will almost certainly be an exchange fee. Like credit card companies, they will also provide a receipt for the transaction. The disadvantage to using one of the national banks in another country is that their fee for the transaction may be higher than that of your credit card issuer. You might also find language a barrier in dealing with banks in another country; no matter how clearly you speak, they might not understand exactly what it is that you want, even if they speak English.
Airports that have incoming international flights will have currency exchange services, usually as a branch of one of the well-established banking firms. Again, the major problem may be related to language.
There are currency exchanges available online. Most will take your credit card information and a courier will deliver the equivalent amount of the currency of your choice--minus a transaction fee--within one or two business days. If you choose to make exchange transactions online, do a bit of investigative work. Some, like Ezforex.com, are established currency brokers but others are associated with travel service or credit card companies. The caveat here is, "Know who you're dealing with."
Large travel agents and agents associated with travel and entertainment credit providers, such as American Express and Diner's Club, have a worldwide presence. You might be able to make your exchange provisions before your departure, whether it be from the United States or another country. Making your arrangements prior to leaving the United States saves time and money when you arrive at your destination. Making currency exchanges before you return to the United States will save you time as you come through U.S. Customs.