I went to Mexico a little while ago. From my personal experience, don't bother even trying to buy them down there. Just bring your own stash. You will have the opportunity to buy cigars everywhere, from street vendors to fancy-schmancy walk-in humidors.

If you are an American citizen, it is against the law to purchase Cuban cigars in any country, Mexico included. When I was down there recently, I figured it wouldn't hurt to see the selection available, anyway. From my short experience, I have learned a few things about cigars and ISOMs. These didn't come close to meeting any of those qualifications. I visited a few other shops and pretty much found the same to be true. I'm not trying to sound crass here, but it seemed like cigars were like any other tourist novelty, such as sombreros, t-shirts about tequila shots, and Corona beer magnets.

Save your cash and bring your own.