you can buy a few Pesos at Walmart in Nogales. However You can Not use Canadian Dollars! They will only accept American Money in the Southern States!
Whatever You do Do Not Enter Mexico without at least $5000 Pesos! I speak from experience. Just when You think ATM No problema.... it will be Either Out of Money or Out of Order!
Also forget about using Canadian Dollars in Mexico !
We use a Scotia Bank Debit Card to obtain A good Amount of Pesos from the Scotia Bank ATM in Guaymas! There is no Service charge to Scotibank Customers!