Your local hardware store should have what you need. They are #10-32 tpi (NF). The screw head is called a "Pan Head" and black ones are "Black Oxide" finish. Get a couple of lengths of 3/8", 1/2", 5/8" and 3/4", just to have around. Take a fore end with, to check that the screw heads fit in the countersunk holes. Allen socket or Torx drive heads might be less fiddly than slots and less likely to slip a screwdriver blade off of.
I think the spacer you are referring to is an O ring that you can put on the inside of the fore end to keep the screws from falling out when the fore end is removed.
Don't overdue the torque when installing a fore end.