Sometimes I feel like an analog guy who's trapped in a digital world. Here's what I would do:

First alternative
1. Take the front derailleur clamp screw off.
2. Test fit to make sure it will work in the rear derailleur (probably).
3. Take the screw to Home Depot to find a replacement.

Second alternative
1. Cruise around the neighborhood on garbage morning. Look for a trashed derailleur bike.
2. Take it home, salvage the clamp screw out of it.
3. Throw the rest of the bike away.
The last step is important because otherwise you'll trap yourself in an endless repeating loop.