Good news,,, I think.
I gave this to my wife, she has a knack for research. Sure enough in 10 minutes she found it. 1/8" sheath by itself in 10 foot sections for $10. It's white, possibly an off white, hard to tell in the pictures. It's expandable to an extent so shouldn't be a problem to fish a single 18 or even 16 or 14 gauge wire through it.
I figure for $10 I'll try it and see. It will be here Thursday. If it looks good I'll go with a new engine harness and put this on myself.