Leitz filters for Black and White Film

Given that an ever-increasing amount of my RF shooting seems to be in black and white (HP5/Tri-x, as well as an growing amount of PanF Plus), I would like to begin building up a collection of filters for shooting black and white (chiefly the green and yellow filters in 39mm, 46mm and 55mm sizes). My preference would be to keep things Leica. The problem? In this, the digital age, filters are getting damn hard to find. So... any suggestions as to where I should look? I have managed to pick up a few from time-to-time at KEH - but that can be something of a hit-and-miss proposition. Any other sources (Canadian or US preferred)?

Hi Bradley,
Marco Schouten in the Netherlands could provide al the filters I whas looking for.
http://www.schouten-select.com/
Hope this is some help.
Kind regards,
Ruben