Determine the currency exchange rate for Mexican pesos at a website such as XE.com. The peso is a free-floating currency, meaning the rate can vary. Compare the current rate to ensure it has not suddenly dropped. On XE.com, click on the chart after converting to view recent fluctuations.
Shop around for the lowest exchange and shipping fees. Examples of websites that sell pesos include Travelex Foreign Exchange and Oanda FXGlobalTransfer. For example, as of January 2011, Oanda FXGlobalTransfer offers a flat fee of $25 to purchase pesos. Most transactions to U.S. bank accounts take two business days or less.
Visit retail or auction websites offering antique Mexican pesos as well as rare modern pesos. Some examples include Provident Metals and eBay. To purchase on the Provident Metals website, for example, mouse over "Foreign Coins" and select "Mexico." Browse the selection of pesos available, review all information and follow the instructions to purchase online.