RSPCA equivelant

There seem to be several. Google:
"société protectrice des animaux belgique"

Maybe http://www.natuurhulpcentrum.be/index.php/andere-opvangcentra

or

http://www.pets.be/dierenasielen

depending on what you want

Try going to the Blue Cross (La Croix Bleue). We adopted a Yorkshire Terrier from there a little over a month ago and we love him! They have so many dogs, cats, and other animals that need homes.
http://www.la-croix-bleue.be/