Any decent upolstery shop should have them. The guy I used to put the vinyl cover on my truck had them by the box full. He gave me a handful so I'd have some in case any of mine got damaged.
Legendary auto
link to catalog Page A130
convertible body
screw studs
5/8 Phillips head stainless steel body screw
studs for snap down boot and well liner applications.
CBBOOTBEZSCRW (Pkg/35)..........$14.00/pk.