Thank you both for your tips!
Stal Hendrix, Stal Bergmans, Peter Bulthuis (sporthorses from holland is the name of his stable) maybe Ben Boessen? Zangersheide also takes interns. First four are in the more southern part of the Netherlands, Zangersheide is of course Belgium (Lanaken), i've heard good things about internships at Zangersheide. You can also look up Kathy Offel, she recently moved back to Germany with her horses and she also takes interns.
Do you mean like a working student type position? You can try yard and groom, they post all types of positions around the world. Good luck!