eBay seems to be the usual suspect with respect to getting one. Alternatively, get a red dot F3 screen of your preference. Disassemble the screen and remove the fresnel screen part itself. Disassemble an F4 screen, take out the F4 fresnel screen, and install the F3 screen in its place. Reassemble, then install in your F4. Enjoy having a split-image again.

-J