http://www.riwire.com/

never bought from them, but they sell various wires with various covers by the foot.

I guess this place died, but I used them before.
http://www.route66supply.com/
page is still up but says under construction. no update since 2011.

this place has 56 color choices if you can't find the trace you want at the top link
http://terminalsupplyco.com/Store/Default.aspx?CAT=GPT&PCAT=113190#Categories