As far as what is used to produce the material, since you can’t create aomething from nothing (Yes, I know it’s a manga and isn’t constrained by rl laws of physics), I’ve come up with a theory:
All Devil fruit abilities stem from the persons chi (haki) or life energy. The material is created from the persons energy, which is why when someone overuses their ability they get tired, or pass out, or go all noodle-ey like Luffy. We know that with enough training someone can use kami-e to reshape their body WITHOUT the need of a devil fruit. So, with enough skill, someone could feasibly recreate Luffy’s gomu gomu no mi through kami-e alone. I think what the fruits do is force your body to do this automatically, on its own. Or possibly forces a transmutation of your haki/chi/life energy into another form, against your will, through an influx of foreign haki/chi/life energy. Since we know that objects can be imbued with haki, weapons and whatnot, this would also explain how inanimate objects can be “fed” Devil fruits. Awakening could be a way to push that same transformation onto other things, since (according to Zoro) everything has it’s own energy, and thusly can be manipulated.
Just a theory, but it makes sense right? I’m not just talking crazy?