You can make one yourself out of steel, or I've heard of others using the heavy rubber that's used in horse stalls, or old conveyor belt.
I went to a place that sells steel and had them cut a 42" length from 1/4" thick by 3" wide stock. I marked the center line of the new steel to help locate the line for the holes, then I clamped the one I was replacing to the new piece to locate the holes and drilled them out. I used a larger drill bit to countersink for the screw heads.
From what I've read in other's posts, the advantage of the rubber edge is it's quieter, and doesn't have the potential for damaging pavement, like black top.