Hi, Liv2 ...
The absolute best exchange rate will be at a bank.
As others have noted, US dollars are almost universally accepted at tourist destinations in Mexico. The exchange rate in restaurants, etc. may not be as favorable as at a bank, but you're only talking pennies. Don't take away valuable vacation time trying to shave a few pesetas.
If you get a check in pesos (say, a restaurant) just ask for the number in US.
pamda -- CM FIrst-Time Cruisers Editor