Blue J Rails
is the only place I know where you can buy em built. They're nasty expensive though for what you get. (Basic 6ft flat would be about $270 shipped.) Just build one. If you don't have welding equipment or don't know anyone who could help you with metal just screw PVC pipes to the top of a wooden structure. if yur talented, you can even heat thin PVC pipes and curve them. Check Here
for some pics of some people who seem to have done this nicely.
'180 net grab = NOT a new trick!'
Have you ever seen a little kid slide back down the inrun of a kicker without going over? This is why parents shouldn't dump their kids off anywhere near the park...