Welcome to the board. There are dozens and dozens of places to catch flatheads in Indiana. Where you are located will dictate which of them are viable for you.
The search feature will open up a ton of info to you...W
Monroe Resirvoire has some big flats in it, and it's a pretty big lake as well.
The white river has some big hawgs in there..
And if you're into the pay lake scene (i hit them up from time to time)- Try Hubbards South Lakes. Their website is www.hubbardsouthlakes.com
if your into winning money, chose a paylake i like wagon wheel always got luck on my side, but i like fishing monroe, just alot of them dam stripers, but i'd suggest monroe, brookeville, strip-pets,potaka, wabash and white river goodluck!
Turtle Creek Reservoir does have some big flats. A guy caught a 47 earlier this month and the flats just came off there nests. Right now the water is about 20ft above normal and has the fish are on slow mode.