The same tire as sold by Andymark (except in black) is used on some smaller scooters and lawn equipment. If it is a Cheng Shin brand 200x50 and has the C-968 tread pattern it will be the same rounded profile, diamond treaded knobby many are still looking for. Our set came from a lawn equipment repair shop.
The black ones are NOT non-marking, meaning your robot will leave skid marks on the floor.
http://www.csttires.com/int/lawn-and-garden/c968/