Why would you need a spare shifter cable if your actual shifter broke? For that matter, unless I was cross country touring, I can't imagine needing a spare shifter cable, and maybe not even if I were cross country touring. Most bike shops have something workable.

I think the best short term solution in this case is either adjust the set screw or loosen the attachment point on the RD and pull some more cable through to get you in a more manageable gear.