They come up on ebay every now and then, but you will need to get it calibrated which in itself is not cheap, here are two in AUZ as we speak.
VINTAGE HENRY BROWNE MARINERS SESTREL COMPASS - eBay, Compasses, Maritime, Antiques. (end time 12-Apr-09 18:24:41 AEST)
VERY OLD COMPASS POSSIBLY WW 11 MEMORABALIA.WITH ARROW - eBay, Compasses, Maritime, Antiques. (end time 17-Apr-09 19:00:00 AEST)
they're are also others on UK etc site, pot luck I suppose if it will calibrate, you can give it a check swing against an aircraft one and if it appears miles off and possibly uncalibratable ( spelling) pop it back on ebay, if not far off the mark from the aircraft one, then send it to be done....... just an Idea.