From my experience:
1/ Lookup where the students did their internships in the previous years. Contacting those companies might be a good option, as they might be willing to take an intern coming from your program if they were happy with the experience the previous year(s).
2/ Try to find companies working on product linked to Operations Research (companies developing Advanced Planning and Scheduling Software (APS), Transport Management Software (TMS), Workforce Management Software, etc.) and contact them to see if they might be interested to take you as an intern.
3/ As you mentioned that you are based in France, looking at the forum of the ROADEF (http://roadef.org/forum) can also be a good bet.
It is not because a company does not publicly post internship offers that they are not taking interns. Anecdotal evidence: when I was a student the most interesting internship that I got to do was with a company that was not advertising any internship. They were making an interesting product, so I contacted them to know if they would be willing to take me as an intern, and I ended up working on an interesting crew rostering problem with them. Another anecdotal evidence: my company is having some operations research related internships available but they are mainly communicated to university with which we already have an existing link (which does not mean that we would not consider somebody coming from another university for them).