Searching for a stage or internship is one of the most difficult things in student life, especially for master students. Here are the list of best websites that can help you to search an ideal stage:
https://www.1jeune1solution.gouv.fr/stages
https://www.cidj.com/emploi-jobs-stages
https://www.letudiant.fr/jobsstages/conseils-stages.html
https://www.directetudiant.com/theme/offres-stage.html
Good luck