About these maps: http://landplats.ilsos.net/ftpofillinois.htm

FOIA: https://www.muckrock.com/foi/illinois-168/federal-township-plat-tiffs-60263/

U.S. Survey General Records for Illinois, "Federal Township Plats," Record Series 953.012, Illinois State Archive. Digital images orginally developed with a Library Serivices and Technology Act grand awarded to the Illinois State Archives and administered by the Illinois State Library.

The raw data files are MrSID format images in the /raw directory. These have been converted to jpeg files in the /processed directory using the mrsidgeodecode program.

You can download all these files here: https://github.com/datamade/plats/archive/master.zip