Can you buy bulk shifting/brake cable for road bikes?
The problem that you'll likely run into is the stop in the lever/shifter ends. I'm not aware of any systems that will accept bare cable at this end:

If you wanted to pair your spool of wire with some sort of crimper (or go all out with some sort of mold/press), you could make your own - but investing $3,000 - $30,000 on some machine that melts metal on the end of the cables probably isn't worth your while.
I don't have much experience with bike shops (I buy most everything on eBay), but I would think they have boxes of these cables packaged in bulk like this: