My understanding is that your best exchange is going to

My understanding is that your best exchange is going to be from an ATM. I would stick the ones outside of Zona. I’ve heard stories of some of the ATM’s in Zona having skimming devices attached.

Also, if you’re going that route, get a card that refunds foreign transaction fees and ATM like Schwab.

If you’d rather just go to one of the Cambios the best rate I saw advertised was across the street from the Rodeway Inn in San Ysidro Blvd. That’s no guarantee though. Some of them get you with commissions and transaction fees.