If you are in Japan, it's cheaper here http://kamoya.info/15_40.html

Two alternative:
1. Cut a piece of an extra shim and stick it below the original shim. With the right length, the seatpost will stop at the desired height.
http://www.sjscycles.co.uk/brompton-...tslv-prod2930/

2. Add a stop screw at the bottom of the seat pipe. If you also chope the flare at the bottom of the seatpost, you can also remove the seatpost with no tools, through the slot in the clamp shim.