There is no need to exchange money before you reach México if you have a sufficient allowance for your ATM withdrawals. The best way to get pesos is through an ATM machine which gives you the best exchange rate and most services do not charge fees from the México end. If they do, the machine will state so. Machines have a maximum limit of $6,000 MXN/pesos per withdrawal, but you can make several withdrawals in succession up to your card's limit. You will find machines in the airports. However, the ATM machines at the airport tend to offer a slightly lower exchange rate. Get only what you need to get by until you find an ATM machine in San Miguel.
The symbol for the peso is $, the same symbol as for the American dollar; the sign for the dollar has 2 lines going through the S. If you key in a $300 withdrawal, you are actually getting 300 pesos, not 300 dollars. All machines have an “other amount” key, and some machines will allow to 6,000 pesos (about 250 - 300 US dollars depending upon the current exchange rate).