Banks are your best bet but often have long lines and short hours. Be observant, however, they may charge a commission. Large hotels exchange money and in some cities especially in the south have ATMs. Some tourist shops and large supermarkets will take your dollars and give you change in pesos. Again, be observan and careful when doing so.
Credit cards are useful in large centers and are essential when renting a car.
Gas stations only take pesos . Businesses may add a surcharge for credit card use. Cash advances are available at banks. When looking to buy that Mexican Hat or trinket at a market or on the beach you will have your fun bargaining there. Most of the vendors will speak English and will enjoy talking with you in a conversation as best they can with most pretty adept to the language.