Get them from an Banamex ATM, at the SuperNorte grocery store across from Sam's club gave me 20.1 -1 on Sat.
ATM Fee was 30 pesos.
I used a no fee foreign transaction debit card from Capitol One.
There are other US banks that do the same, Bank America does from their Mexico partner banks. I've read.
If you are going to actually exchange $$ for pesos the Electra store had some good exchange rates last time there.