Mountain Dew in Belgium

Any supermarket with all the other sugary soft drinks.

Most of big Carrefour supermarkets have "international food" section.
I have seen Mountain Dew in such a section at Carrefour Mont St. Jean Waterloo
https://magasins.carrefour.eu/fr/s/carrefour/hypermarche-carrefour-mont-...

I have seen it in the Carrefour at Thiefry

I have also seen it at some Esso petrol stations