My supply has dwindled, but Google came to my rescue, as estate sales have been a bust lately. Here seems to be a source for plain and black oxide slotted steel screws. I'm fixin' to order some, so thought I'd share the link. They have been around for a few years, but seem to have found a supplier as inventory levels are a lot higher than ~5 years ago.
https://blacksmithbolt.com/pages/screws
https://blacksmithbolt.com/pages/screws
