Probably won't find any Cantillon actually in Luxembourg but if you drive just across the border to Arlon Belgium you will find a lot of it at MiOrge MiHoublon. You can find Tilquin, 3F and some others at Humulus a shop 15 min North of Luxembourg City. Just make sure to check the hours of the shops as they are very limited. Other option would be to do an IP trade ; )
When I last shopped for beer in Luxemberg city, I could get Cantillon organic Geuze at a small Delhaize city close to the main train station. For around 3 Euros (0,75l). I fear those days are gone. (I think it was in 2006 or so...)
