I think what you're looking for are called cable grippers. But unfortunately I haven't found a place online yet that sells them for very cheap. Your other option, yet not as sleek, would be aluminum ferrules or stops that you just clamp down w/ vice grips or a swaging tool.
i need the top half of the hardware to be adjustable, where the loop is.
My fixture came with the cable to hang it, and instead of the cable just running through a hole, it has a threaded fitting on the end and threads onto another threaded fitting coming off the fixture.
Well, heck, you could always use rope. tie or clamp it to the fixture, run it up over a hook or pulley, and down to a cleat (http://en.wikipedia.org/wiki/Cleat_%28nautical%29). With a couple wraps, they'll hold a boat in place if anchored securely. If you wanted cheap and adjustable, that's an option. Unfortunately I don't think it will work for wire.