We have always used Duck Canvas. Jo-Ann Fabrics carries both a vibrant red and a royal blue that are quite close to the FIRST colors. It's heavy enough, tough enough and stays nice looking throughout the year. The vinyl numbers also stick very well to it. The total cost for 48" (by 60") of both colors was $13.
To see the red bumpers on the robot go to our home page:
http://www.firebears.org/