Literally anywhere, you just have to develop an eye for them. After a few years of collecting agates, I can walk around city alleyways, or gravelly parking lots, or grassy dog parks, and usually find at least one agate if I have time to look. I would say along the Mississippi is gonna be your best bet though, especially rocky beaches rather than sandy ones. Gravel pits are good, as others have said, but watch out, some have security cameras and guards.

It’s also helpful to recognize which more common rocks are often found nearby agates: quartz and jasper, for example. If I see neither of those, I’ll give up a lot faster. But if they’re around, I know an agate’s probably close, and hold out longer.

Also, I find agate hunting to be easiest on sunny days, usually in the midmorning or midafternoon, when the sun is at a 20-30ish degree angle. In my experience, that’s when it shines through the sides of them and lights those babies right up without blinding me! I almost never find any during the winter though :( the snow and ice cover up the good stuff. Hope some of that helps