I've purchased "short" lengths of SO cord from this site before: http://www.skycraftsurplus.com/heavyduty-so.aspx?page=2

I say "short", since most places that sell SO usually have a 200' or 500' minimum. This place will sell 50' or 60' minimums. SO is a rubberized, stranded conductor extension cord that is waterproof and very flexible, yet some what expensive. It's way better than NM/UF options you have at the warehouse stores.

Mike