Mcmaster carries a wide variety of wire, and tends to be cheaper the digikey. We do most of our wiring with McMaster Zip Wire (9697T4 for 12 awg) which is more flexible then the KOP wire, but may or may not be flexible enough for your application. They also sell silicone wire for about $1.50 a foot.
Another option well known by RC car people is Dean's wire. They call it "wet noodle".
http://www.robotmarketplace.com/products/0C-WSD01R.html