They have been available at times through some vendor's and seen them at gun shows. Availability seems to come and go though so when you see one you need to be quick. I have 2 USGI stocks, one wood and one syn. I bought the wood stock at a gun show and the glass stock I bought from a store.
You can find (sometimes) a dummy selector switch to fill the cutout on the wood stock and JB weld works well to fill it on the glass stock. FYI a USGI stock may take some fitting to work safely on a Norc M14. My Norc had hammer follow through problems before fitting.