Color me Jelly. Lived in central Indiana, but 2 decades has past since.
There are a ton of GREAT lumber resources in northern Indiana. There are even more sawmills in southern 1/3 of state, if you every get near bottom portion of state.
TBH - Porter county and NW corner of Indiana is mostly industrial wasteland and suburbs. The forest have been plowed under decades ago when the area near Chicago was big into steel processing and fabrication. You will have to drive 1-2 hours to find sawmill and inexpensive lumber sources; but the results are worth every minute of the drive.
+1 Frank Miller. IME - Largest and coolest collection of QS lumber in world.
Forget all the names, but sawyers scattered around; La Crosse, Plymouth, Elkhart, and Middlebury Indiana (towards the center and center-east) part of the state.
Middlebury has several wholesale lumberyards, and if you need a 300-500bdft unit of lumber; the prices will be cheap compared to retail. And it you can tolerate short ( < 8 ft) lumber, the deals are even better.
While it might sound far away from the Indiana/Illinois arm pit; also have sawyers NE of you near Galien, and Decatur in Michigan. There used to be several small sawmills in Edwardsburg MI, just across the border from South Bend IN.
Finding these folks is easy:
Ask Google Maps, "Sawmill near South bend, IN"; then expand the map slightly, and click on 'search this area'; you will be rewarded with a plethora of saw mills in northern Indiana, and southern Michigan.
PS - The NorthWesdt wood workers Assoc, and the Central Indiana Woodworkers club are fairly active. They used to maintain a list of lumber sources, and even negotiated club discounts for members. Couple of the small sawyers in Northern Indiana used to be club members, and the member lumber price was usually too low to be discussed unless whispering in dark corner.
Best Luck.