Fish for sushi?

Yes, you absolutely should only use frozen fish because freezing kills all the parasites. Nematode worms like anisakis, as well as D. latum tapeworms etc. You can get good frozen fish from Picard, but also Colruyt.

It's not about being reputable. Fresh marine fish are wild animals, they have parasites, just like any other wild animal. Freezing, or cooking, kills the parasites. The risks of humans getting these parasites is relatively low, but it is a risk.

It's exactly the same as pork. You can go to the best butcher in the country, but there is still a risk of getting tapeworm from uncooked pork.

sushi quality is normally the superior quality, you are correct and should be bought and prepared the same day.
I would go to Rob's or Le Marche des Chefs, Rue Lens (Ixelles) - all good fish is pricey!
Bon appetit, xl

You're probably best walking around Place Sainte-Catherine, Quai aux Briques. Loads of fish restaurants and fishmongers in the area. It is the old fish market.