OK, I'm in the US contingent, but...

Antique & Vintage Firearm Network
Hallowell & Co. (they have a Lancaster for sale, might contact them about sources)
I'd say there's also a chance that the company might still be around, under the name "Charles Lancaster & Co.", as there's a book on hunting published with them dating back to 1962. I can't find a direct link with a cursory search, but I hope you have better luck.

Hope this helps...

M