Get an empty Coke can and cut a piece out of it. The metal is very thin and you can cut it with a scissors. Makes great shims and they are thinner than most brass you can can find.
Also, if the sight fits well, has no up and down movement, and just slides back and forth in the dovetail with no slop, you can use a drop of Red loctite. Once its set, it ain't gonna move.