Dumas has their mechanical one. Vantec sells nice ones, but they're pricey - like $150 or so, though they do sell one that can handle two motors on separate channels. Robbe, Graupner and a couple obscure British companies have them, too.....

Old River Bill Zumwalt has drawings on his website to build one, too. I don't know his site, but 'old river bill' in a search engine will find him easily enough. GREAT website.