M1900 are not so uncommon but then again not that easy to find... you may have to travel a bit. The best way would probably to go to a gun show:
http://www.swissguns.ch/datebourses.htm
Lucerne may be an option you want to consider.
Otherwise you may try your luck with Geneva gun dealers:
http://www.swissguns.ch/armuriers.htm
Possibly "Mayor Ernest SA" or "Armurerie des Bastions SA"
Prices: CHF 2-10K depending on version, condition and bargaining skills.