Should be 1/2" thread size. Don't remember if the hex is 3/4" or 13/16" on the stockers, but that might not matter except for a religious points-centric restoration.
I would think that any wheel place could supply some. I know I have a few old ones laying around from before I wanted fancy, but they would not be very nice condition for new wheels.
Check with a local tire store to see what they have.
Otherwise, even though not stock my go-to lug nut is a "bulge seat" open ended nut which is available just about any supplier.
I just like the way they look.
Paul