Hey there,
I guess this falls under the methods flair?
It depends on where you are. For instance, I'm in the Drome right now and the trial files for the epuration are in Grenoble. They've actually already been released. In some cases, depending on where you are, you'll need to receive special permission from the archival director to look at some files. This means you have to build a relationship with the archive you're going to research.
Locate the departmental archive in the region where you want to work. There you will find thousands of files pertaining to individuals under investigation, not just trial notes. For instance, the local police, gendarmerie, and police nationale have all conducted interviews for persons of interest both before and after liberation. These files provide critical information on the lives of the people in the region and their activities during the war. Of course, as some were suspects of collaboration (or before liberation, of the resistance), they have an agenda to free themselves. Nevertheless there's a lot of material to be gained there.
In addition, visiting the municipal archives of the locality you're looking to study provide similar information, and really get into the minute details of life in these locations.
Lots of other investigation files are at the French Army archives in Vincennes, and you can go to the Archives National in Paris to find other important judiciary files. To start, though, I'd check out regional departmental archives where you're researching, because for many accused their trials were held at these regional courts.