Craiglist. I've seen a bunch of stuff on there.
Actually bought a rail off there to start our season off. Didn't have a working welder at the time and I got it for cheaper than the steel so I still think it was a solid deal. Ended up hacking the legs off and welding on new one to space them properly, but we had that rail in as it was to start the season off until the first weekend in march.
Or buy some square of rectangular tubing and have a friend or shop weld it. Or buy a shady welder from harbor freight for 100-200 and weld them up yourself. Maybe ask for welder, grinder, and some steel for christmas.