I have never been to Mexico, but I have read that in Mexico the banks sell silver bullion (Libertads). From what I read they have premiums on them similar to what you might pay here. Don't know if that is true or not. Silver is generally considered valuable in Mexico by the populace unlike here (in general that is), so I doubt you will get lucky in the CRHing sense down there like you can here. But it may be worth trying if you go. Don't know about getting your bullion through customs on the way out.
I hit a lot of yardsales in SoCA so I speak with first hand experience. The average illegal Mexican knows more about silver than the average American seller.
My Spanish is not great and I have to work hard to buy a silver ring from a Mexican but the same ring from a soccer mom would go for .25¢