Where to buy sequins, glass-beads, embroidered flowers... for fashion design.

Please try at "Veritas". You will find a shop at every city like.

Crea, Tomberg. They sell all sorts of items for crafts.

Sometimes Hema have a few bits and pieces in.
If it's more beads there is a Jewellery making shop in OVerijse but can be a bit pricey depends if you're after cheap and cheerful. Then creacorner definitely they have more than one, Stokkle, Waterloo and Wavre I believe
http://www.creacorner.be