The most prominent EU supplier is Tost. They have various tow hook arrangements for various aircraft. Suggest you start there.
Tost Flugzeuggertebau
But why go for a hook? Most glider towing aircraft I have seen (admittedly not a lot) have a retraction system where the tow cable retracts on a drum in the baggage bay. It's far more convenient since you don't have to drop the cable into the field from a sufficient altitude, then circle around & land, and don't need to have someone retrieving the cables from the field. Simply hit the retract switch after glider release, land, park in front of the next glider, pull out the cable, connect, take up slack and off you go again.