Channel Master item number 3084 (100 feet of 20 gauge 6-strand galvanized steel wire) is available from their web site for $12.99 (
link), and they have a free shipping promotion going right now. This kind of wire is only good for holding 100 pounds or so, but that should be sufficient for mast stabilization.
It's probably a lot cheaper than the typical galvanized or stainless steel "rope" you see at local hardware stores that can hold 700+ pounds and costs upwards of $1 per foot. That stuff will work too, but it's overkill for this kind of application.