Bikes at Work sells just their trailer hitch, but it's $50, and if you can weld you can probably make the same thing for about $12
Anyway they also have a cool bike-trailer
calculator that shows you how much you can supposedly carry (in lbs.) on a bike trailer.
And there's even a pic of someone hauling 1000 lbs of cargo on three trailers with one bicycle. I think they used to have a video of the 1000lb haul, but I can't find it anymore.
