Rhode Island Wire as GW posted, good people to deal with:
http://www.riwire.com/

I've used YnZ's before but their online catalog is very sketchy
since they changed it, the print catalog is more complete:
http://www.ynzyesterdaysparts.com/

Wiring Products.com has a complete line of asphalt coated wire loom:
http://www.wiringproducts.com/asphalt-coated-fabric-loom

Some of the vintage automotive vendors also carry it in a few different sizes:
http://www.classicparts.com/1941-87-Wire-Loom-3_8-ID-Foot/productinfo/52-956/#.UvyqmsuPLcc

DG


Denny G
Sandwich, IL