I take the bus down to the grocery store across from the riu los America or close to it to get my miller lite and they have a wide selection of beer and other stuff there, and not that bad of pricing either.
It was funny that they actually had it for 1 day at the premier members bar. They were actually the little 7oz coronitas. The owner of the resort had his birthday party there the night before, and these were left over. We saw them in the fridge, and then just kept ordering them and drinking them extremely quickly. The case didn't last very long at all.