Your first, and best source, is your local B&M. Yes. The local music store. Antwerp is a large city. There is more than one. Talk to all of them. Find out where the local pros shop.
The second source is the local pro scene. Talk to the guys who are making a living with their guitars. They will know the techs. And don't restrict yourself to bass players. There are ten times as many guitar players out there. Statistically, that means many more will have had fret work done.
Third source is the local music paper. There is probably a small newspaper that covers the local scene. These guys advertise there. Pickup a copy.
BTW, a quick (less than two seconds) google search of "luthier + Antwerp" reveals a young luthier who is attending the
International Luthiers School and
Centrum voor muziekinstrumentenbouw. Certainly they can give you some direction.