Cameron, half inch #8 stainless steel self-tapping screws from the local hardware store work for me. You will need hundreds if, like me, you replace them inside as well as outside. I've found it more economical to buy complete boxes, rather than loose. I have not found galvanic corrosion to be a problem, but, if you are near the ocean, a little zinc chromate paste or Sikaflex on the threads would do no harm.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."