Security equipment for Rollerbalading etc.

Decathalon is a large chain they cover a range of sports so they will probrably have something but choice maybe limited because it's a one stop place so
Won't always have a massive range for all sports.
If you drive the one at wavre is very large never been to the other you maybe able to see on line what they have I'm not aware of any specialist shops and imagine they would be pricey here

Try Billy Kite (http://billykite.be) 100M off Place Flagey.
They have head and limb protection, site shows prices.