vertimax.com sells them. Not cheap but they work great. you can literally apply restiance to just about any movement you can imagine. I work out on the thing about twice a week. I hate it because it sucks when your on it because it literally makes you wanna puke and just burns, but i love it becuase you get results quick. It put some inches on my vertical and i'm dunking a basketball fairly easy at just over 6' feet tall.