Few more to add...
223 avenue Georges Henri, Brussels
Woluwe shopping centre, Brussels
Club paper shop (city2 and a million other places)
Bit expensive but it's a general stationary/art supplies kinda place.
Zeeman, Wibra, Kruidvat and Hema (Wavre, Leuven, Brussels) often have little bits for kiddy or simple crafts too.http://www.creavenue.com
has a craft/art/classes search thats pretty extensive.
For dress material the only place I've seen thats any good is Pauli in Leuven. Everywhere else is either unreachable by public transport or way to expensive to consider (if anyone knows different please please let me know...my UK supply ran out months ago and withdrawal symptoms are starting to kick in