Currently there are 44 native Telephoto Prime lenses for Canon 50D and 33 of these lenses are autofocus (AF) type.
Canon 50D doesn't have a sensor based image stabilization system but 18 of these Telephoto Prime lenses features optical image stabilization.
Since Canon 50D has environmental sealings (Weather Sealing), you may like to couple your camera with one of these 21 Telephoto Prime lenses with Weather Sealings.