Summary

It's really easy to change the shifter cable on Shimano trigger shifters. All you have to do is take out the little plastic plug and you have a straight shot to pull out the old cable and feed in a new one. Then you just have to screw the plug back in, as soon as you find it. It was here a minute ago. Did it fall on the floor?
It's not that it's so expensive to buy a genuine Shimano replacement, it's just inconvenient. A trip to the LBS just for that? Or mail order and wait a week (and pay shipping)? If you print one you'd have the replacement pretty quickly. And in whatever color you'd like (maybe even glow-in-the-dark, so that maybe the next time you drop it, it will be easier to find.) And, while we're at it, although the Shimano original can be installed and removed with a phillips or flat blade screwdriver, a 4mm hex drive might be nice option, so I included files for both versions.
The original Shimano part appears to be nylon. I printed one with ABS and it seems to thread in and hold well enough.