my first question is where are you staying at?
I Just came back from there about two months ago...IN dowtown Cancun there is a place called the flamingo Mall where I bought both my ashtrays...every cigar I saw in these places looked very fake. IN the Resort I stayed at (Moon Palace) in the gift shop they had a small humidor which was full of tasty ISOM Cigars..as well as someone on site handrolling some nice smokes as well. If you go to Isla Mujeres while you are there...avoid all the smoke shops at all costs..these fake cubans make fake ISOM"S taste like Dutch Masters. So your best best (A bit pricey) is the gift shop in your hotel... I had to pay almost 20 a stick for some Montecristo no. 2's while I was there..but they were worth it...the handrolls were about 10 a piece..and I brought a box of those back with me since they had to bands on them
there you go
Also In Cancun there is an Official ISOM shop ran by the CUBAN GOVERNMENT where you can get legit ISOM cigars. I am sure your Concierge can tell you how to get there