You mean flyball? Where the dogs run over hurdles and hit a large platform which ejects the ball and they ctach it and run back? Or there are just the things that you can buy...here are some pics

This is flyball when they are ready to go back.


Here's a contraption...


And a home-made one


I'm sorry, but I don't know where you can buy one. But there are some websites that you can get instructions on how to make one...