One of the biggies is CableCam, but I believe that they don't sell their product, they rent themselves out. I went to NAB in Vegas last year and found another company called Noblet Enterprises, who have the Noblet Camera Trolley, and I believe it's for sale, not rent. You can check out their website at http://www.nobletproductions.com
Allen Film & Video
"Who's the more foolish? The fool, or the fool who follows?"