Where does Greasemonkey cache scripts that are added with the "@require" keyword?

GM stores all of a scripts data within it's own folder within the gm_scripts folder residing in your Firefox profile folder. To find your profile folder please read this page.

So to find the @require'd files for script xyz look in: %Firefox Profile%/gm_scripts/xyz/

That's no longer true with recent Firefox - check this answer which tells that they are hidden away in a sqlite database.