Lee W. Pedersen tractor restoration in upstate NY. He's got a few different colors and patterns available too.
Our very own Alliance vendor The Hot Rod Company has them here: http://www.thehotrodcompany.com/ Look under FlatHead parts and accesories
I've heard of just getting a roll of shoe-sting (it's braided cotton) and dying it the color you want. Just thread your modern wire through it.
i'll second the hot rod company, i just got a set for my small block. theyre green with yellow tracers theyre the only place id seen with it. otherwise check http://www.magnetoparts.com/wire.htm http://www.riwire.com/
Sacramento Vintage Ford has them as well. The original Ford ones had four different colors to denote which cylinder got which wire. If you ask them they will make you up a set in any of the 4 colors for all 8 wires. They are pretty cool people. I don't think they are Alliance Vendors (yet) but they ARE awesome to deal with. Tim