I commonly see the same brand of canned Thai coconut milk that I buy in the US, most frequently at Mega.
As with every other commodity, they have it when it's in stock and don't have it when they're out.
PACSADeli has a website and supplies restaurants. Why not contact them and see if they can get several cases for you prior to your arrival? The Cozumel phone number is +52 (987) 869-1935 and their email is firstname.lastname@example.org.
There are several people on the island making and selling organic coconut oil. Fresh coconut milk is quite perishable, so I doubt they usually offer it for sale, but with access to the equipment needed for making oil it should be easy to make milk. It's possible that if they have a reliable customer they may be willing to make it for standing orders. In parts of the world where coconut milk is part of the cuisine, it's often made at home. I'd check on the Facebook group Cozumel 4 You for starters since I've seen several discussions there about local coconut products.