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Stickers "no pub"
I think Ava in Jesus-Eik have them and others; I would suspect places like Club would as well.
Don't spend money on them... all you have to do is send an email to Bruxelles Environnement including your name, address and however many stickers you want.
http://www.bruxellesenvironnement.be/SendMail.aspx?MailID=75&langtype=2060
I just got one a couple of weeks ago at Papeterie Leopold http://www.parcleopold.com/
BRICO sells all sorts of stickers.
You should be able to get them at your commune - that's where I got mine.