I have a big solution and there are many *.cs-files that actually don\'t belong to my solution (not included to csproj-files) any more. Is there any way to find all of them
Use visual studio to add all files to source control. It will only add files that are part of a project so the non-project files will not be added. You can then simply commit all files and check the project out somewhere else. Only the relevant files would be checked out in the target location.
Given that you have a large project it is of course unlikely that you haven't already got some kind of source control, so you may have to break the existing connection, clear the original source location after the checkout to the new location, copy the target to original and let the originals scm detect the file removal in the original and submit the removal.