Currency exchange is never a flat ratio. Besides the fact that rates are constantly changing there are two distinct rates, the buying rate and the selling rate. If about 10 pesos were the equivalent of one U.S. dollar you would probably trade that dollar and get nine and three quarter pesos in exchange. If you were to take that same nine and three quarter pesos and exchange it for dollars, instead of getting one US dollar back you might get 95 cents. This does not even account for any fees or commission that you may be charged. Because of the fees associated with trading you don't want to make too many trades. Because of the money you lose in the exchange you also do not want to wind up with more pesos than you need. Estimate on the low side for pesos that you'll need and then exchange more dollars to supplement any extra needs. There are lots of ways to trade in more dollars for pesos and some countries like Mexico that use pesos also tend to take dollars.
You can exchange American dollars to Mexican or other pesos in many different places. In large U.S. cities, major banks and travel agencies offer currency exchanges. If you are traveling to Mexico most towns there will have a bank willing to exchange dollars to pesos. ATMs are also more prevalent and you can withdraw money from your bank account while in foreign countries and it will be converted and dispensed in pesos. You should probably make sure in advance that your bank is part of a network that works with ATMs in the country where you will be traveling. Travelers checks are used less often but they are a secure way in which you can get your dollars converted to pesos. Travelers checks can be taken to a currency exchange in Mexico or other countries and you should be able to get the dollar amount converted to pesos with no fees.
