If you so very want some mylar over carbon fiber put up in L1, and not ever launch that from Earth then Luna is the most obviously cheap and abundant source of whatever. No need for asteroids at all. Also the comms delay is 1.25s IIRC.
I personally consider this a folly.
On the other hand, no comprehensive survey of Luna was ever done, and we target Mars or even asteroids why? I'd like some at least plausible reason for this.
It is true that Luna is halfway to Mars in dV on hohmanns. But not in time spent. Never will be.
Luna has the aluminum but probably not the stuff to make mylar or kapton or something similar. You need CHN (Carbon, Hydrogen, Nitrogen) for that.
On the other hand, O'Neill's students did think a bit about how to make metal films without any plastic backing and that might be (1) practical w/ Lunar materials assuming you can get the mass driver, catcher and all of that working, and (2) produce a very high performance sail if it survives the corrosive space environment, metal-Kapton sails didn't do that well here
https://en.wikipedia.org/wiki/IKAROS