Ebay item #4606833664. 100ft, 3 conductor. A little thicker gauge than you wanted (22ga, not 24). Also, the wires are silver/teflon, but the shield is tinned copper. I wouldn't imagine that would matter (who cares how the electrons make it back to the source, it's how they get there that matters).
Other than that, try worldwidewire. They have a good selection of surplus wire at good prices, though they can be pretty flaky.