Where can I find proposals from companies to work with them? Are there lists of such proposals available somewhere?
I am not aware of any such, but around me, several mid- and large-scale projects are done in a very close cooperation with industrial companies, also on machine learning. Even I am employed as a post-doc on such a project, from a large part financed by a company who is also the supposed user of the results. At least in the western EU, this is a frequent model in computer science and related technical disciplines.
One way, or another, to complete a master thesis you will need a supervisor from your university. In the case there are industrial cooperation projects in the area of your interest at your university, you should speak to the local project consortium member, basically one of the professors, or senior researchers at the relevant department. To find out whether such projects do exist at your university is again something to speak about with your local professors/faculty/research staff. After all, at least in places I am familiar with, working out a master topic without a tight connection to a project/theme at the department of your university is not the way to go, most probably even almost impossible. After all, you need a supervisor and the person needs to have an interest in the work. But of course, maybe in other parts of the world than I am active, things tick differently...