Some early Freikorps insignia was off the shelf from existing stocks. A good example of this is the oakleaf collar badge worn by Freiwilliges Landjägerkorps, a readily available insignia worn by many hunt clubs in Germany. I don't think there is any currently available information about who manufactured most of the specialized Freikorps insignia. I assume that information originally existed in the Freikorps archives which were destroyed in a bombing raid in 1944. I would guess that manufacturers that supplied Imperial uniform insignia made some of it. Some well-known manufacturers like Deschler, Godet, Wagner, Fleck, AWS, Timm, St&L and Beco made Freikorps insignia. Deschler, for example made most of the Bavarian Freikorps insignia. Heinrich Timm was a major manufacturer of both Freikorps insignia and awards. But generally it's hard to know who made a lot of it.
Here is a generic insignia catalogue from the period showing hunting and gun club insignia adopted by Freikorps units.