I
think the most comprehensive website is www.immoweb.be
but there will lots of houses/appartments that won't be on there...
the other thing to do is trawl through the goudengids for different real estate agents and approach them individually
http://www.goudengids.be/search/leuven/immobilien.html
or pick up a local paper from the neighbourhood..
http://www.passe-partout.be/RealEstateNL.asp
That's the how... the where is up to you, is Interleuvenlaan in Heverlee... ?postcode for Heverlee is 3001... Lovely part of the world in the main (I live there) but some bits aren't as nice as others... in general though Leuven is a great place to live ...