I've never hunted muskrats, but I have trapped a bazillion of them back in the day. They live in almost every body of water from little branches to large rivers and lakes, especially if there are good food sources like crop fields or cattails and other marsh vegetation.
You mean what area in general? Pretty much everywhere I've ever been from the southeast to the midwest where there's water, I see muskrat sign. Look for droppings on rocks in the water, slides, dens, tracks in the mud, and places where they've been cutting and digging vegetation.