I bought new 37mm yellow-green and red-orange Heliopan filters, before they stopped making them in slip-on mounts. The rest I found on eBay. Mostly Zeiss, Ceneiplan and Panchromar. So now I got:
Very light yellow (Sportfilter)
Some are in the Panchromar "Wechselfassung", so I can easily swap the glass if I want. Filter glass can be bought new from B+W to fit.
This is a cool site: http://photobutmore.de/vintagephoto/filter/ It's in german, but has LOTS of pictures.