If you stay with traditional fish finders, either a Humminbird or Lowrance unit in a 5" screen will give you basic 2d sonar and ability to see cover and fish below the boat and fit your budget. A 5" screen is pretty small but definitely useable. For new units you are probably out-budgeted in the 7" and up screens. There are some places that sell working used units they take on trade ins, but those places vary wildly with having stock of the cheaper units and they go quickly when they are in. The Lowrance Hook 5 is on sale at Cabelas/Bass Pro for $200 right now. Ideally you could get to a sporting goods store that has some available to preview, but I don't know if that's possible or not.
