Just about every liquor store on Cozumel sells it, but I found it for a good price at Europea, that wine shop across Melgar from what used to be Margaritaville.
The best price I've found for d'Aristi Xtabentun has been at Aki. The best price for XO Xtabentun has been at SEDENA. I prefer the d'Aristi since it has more flavor of the xtabentun itself. My dad prefers the XO because it has more honey and less anise - I find it insipid.
I don't know if El Moro really has their own house brand of xtabentun or if it's just a less-common brand. I do know that years ago when I first had it there it was d'Aristi brand.