This is actually for construction and building supply shops don’t sell anything of the sort. I’ve tried:

• pharmacies: their biggest needle is not quite big enough. The substrate clogs the needle and there’s no way to clear it because wire thin enough to push through is too soft and bends too easily.
• pet shop: they don’t have needles, or at least nothing big.. nothing for horses, for example.
• cooking shop: Intl. Home of Cooking has a quite fat needle for injecting marinades but it’s really short/stubby. So it’s too short and crazy price (€17).

I would think a needle for a horse would be ideal but pet shops in Brussels have nothing for horses.