Farm near Brussels

if you type "farm" in the Search function you'll come up with some results.

http://www.mariesgarden.be/en/mariesgardenbrussels
this was advertized some time ago. never tried myself but i am planning to go. however it seems is going to be closed until next spring

If you want bio vegetables you could also look into the delivery systems of http://www.lespaniersverts.be/ and www.agricoverts.be. Tried both and liked them both. Different drop off points throughout the city...
There is also a bio market every Wednesday at Place St Catherine and every Thursday at Place Albert.

Bio-market
Klarastraat 2, b1
3080 Tervuren

http://www.bio-market.be/en/index.php?PHPSESSID=b8585ecd0e2272b1b9528524...

colruyyt buy much of the organic produce in Belgium, try there I think Maries garden is shut for the winter. `you can register for their website. newsletter.

La Finca, Ch de Malines Wezembeek-Oppem. Tram 39 Ter Meeren stop; just after the Citroen garage. They sell their own farm products.
http://www.la-finca.be/

I've been super impressed with the quality and variety from Heureux Nouveau, they even delivery weekly baskets to your home or office: http://www.lheureuxnouveau.be/fr/

I use them: www.reason2.be