Go to a bike shop. Don't worry so much about the brand - you might find something from a company you've never heard of that works great, and you'll find the shop is more important than the brands they carry. You should be able to get a good hybrid/city bike/urban bike for under $500 fairly easily. Moreover, since these bikes are designed for beating around town, carrying groceries, etc, they should have no problems with your weight.
You are at the edge of the Ozarks which can get REALLY hilly. You're going to see internal gear hub 3-speeds and 7 or 21 speed deraillieur bikes in your price range. Avoid the former and try to get something with a Megarange gear (giant granny gear on the back) or a 21 speed, which will have a small inner chain ring giving you some low gears. If you stay in Springfield you shouldn't have much problem, but once you get into the countryside you may still find you have to walk some hills. Don't get discouraged.