Last ones I bought, I got at a gun show. It was all I could do to pay the guy for them, he was more interested in hearing what I had in mind, than he was in seeing even enough to buy a cup of coffee for them.
According to the results of an ebay search, 8-40 screws are used on a lot of Marlin lever guns.
I would suggest if you cannot find a gun shop near you that does any business in scope mounting, look up the folks at Gravel Agencies (Agency?), the main warranty folks in Canada for a bunch of different makes. Takes a lot of those plug screws to fill an envelope!
Cheers
Trev