The only adjustable spacer I've ever uses was Easton's "Bear Trap" which was adjustable over a small range and used to set headset preload in place of a top cap bolt. It didn't work at all well and tended to vibrate loose during every ride. I very quickly replaced it with a normal expander plug and top cap. The Problem Solvers adjustable spacer fietebob linked to works similarly but seems to be a more reliable design.

Actually building a conventional spacer stack to your exact requirements should be easy as they come in a wide variety of thicknesses starting as thin as 2 mm and up to 40 mm. Also, the gap between the top of the steerer and stem isn't that critical as anything in the 2-3 mm range is fine. You can, and probably should, have a spacer above the stem to assure the steerer is fully within the stem's clamp.