where to find a certain kind of rum...?

I would recommend looking at supermercado Nacional in punta cana village. They have a good variety there and I know they sell other products made by oliver & oliver, I cannot comment on price or availability just know that would be the first place I would look. It's quite far from your hotel, if you are leaving by taxi to go to the airport you could probably stop there on your way at no extra cost.

Super Pola in the San Juan shopping center has it. It is about US$50

$50 for rum?!?!?! It must be rum from PR to be so expensive.