Known as a "brocante" in French, the Waterloo flea market is held every Sunday morning rain or shine (usually rain). You can buy just about anything there as well. Sometimes the prices are over the top (makes you wonder how somebody can ask for 100 Euro for an empty bottle) but you can also get some great deals.
It starts around 7 a.m. and goes on until rought one in the afternoon.
Have a coffee and burger at one of the food wagons and then buy an old typewriter!