Are there any secret exceptions for tourists buying alcohol on Poya days?

There are absolutely definitively zero legal exceptions or secret loopholes for tourists. The nationwide ban is a deeply respected cultural and religious law that applies strictly equally to every single person on the island. Any local shop illegally selling alcohol on a Poya day risks severe massive financial fines and losing their operating license permanently. You must simply respect the local custom and enjoy fresh coconut water or local tea instead for the day.