Fan of sour beers like Boon and Vicaris and looking for a good selection of sour beers in or around the Brussels city centre
What is an easily available sour beer in Brussels
Accepted answer
Boon's Oude Geuze Mariage Parfait is easy to find and a good benchmark for a lighter sour. For more traditional or intense stuff, stick to 3 Fonteinen or Cantillon. I usually hit up a certain spot for their selection or find some good sours on tap at a place in Schaerbeek.

Replies
I had a Cantillon Fou Foune last week and it was sooo good.
I actually find some of the traditional lambics a bit much. They can be intensely sour and quite challenging for people not used to the style (even for me sometimes)