it is against US law for you to buy them. Mexicans will gladly sell you anything. When I was in Cancun, I found a few store where you could buy ISOMs that looked pretty legit to me. I read the guides before i went and they seemed to be legit to my novice eyes. That being said, there are also a lot of shops that sell blatantly counterfeit smokes. Don't ever buy anything that comes in a wooden box with a glass top.