Go to Napa and buy what you need, they should have them individually.
Used oem ones are usually rounded off and don't look very good even
though you are probably going to put a center cap over them.
Rally wheels use regular lug nuts like steel wheels in case you didn't realize that.
The second set of lug nuts were also used on the Chevelle SS wheels (they didn't have the bulge above the seat, but I couldn't find any 7/16-20 lug nuts like that). Just make sure your studs are the original 7/16-20 thread.