Unistrut. You can get
Unistrut bolts ("spring nuts" to some, because there's a spring tack-welded onto the
nut) from larger hardware stores, McMaster-Carr, Grainger, the regulars.
City Theatrical or
The Light Source make a
Unistrut adapter, too, if you don't mind paying their prices.
Unistrut bolts come in a variety of sizes (both in terms of the
bolt diameter and the size and shape of the washer). I would go with 1/2-inch bolts because they're more interchangeable with "normal" lighting clamp hardware.
Most people learn to hate the stuff very quickly. It's difficult to tell when the
bolt is actually locked into place, and if the
nut spins ninety degrees (which is easy to do when it's not locked) the
unit will fall. Unlike a
c-clamp, which usually holds the better portion of the weight of the lighting
fixture, the
unistrut bolt won't hold any weight until it's locked down. Don't even try to hang something like a mover or
striplight that requires more than one hanging
point without having thought through the process with a friend very carefully. Because the
unistrut is attached directly to the I-beam, you can't wrap a
safety around it the way you would around a pipe or
truss. Similarly, you can't run cable along the
unistrut unless you get a lot of eyebolts.
But, it's cheap, and goes right up to the ceiling for a few more inches of height.