Where can I buy PLATE SCREWS foy an ALEMBIC EPIC 4 strings THANKS

Hi Fernando - why not try the online Alembic store?

per the FAQ: For truss rod covers and backplates, the inserts are very small 2-56 size
 
so, the screws are 2-56 size, that is a number two machine screw with 56 threads per inch. you can check the length against one you already have, unless they are all gone.