The LDMCC has a web site at
http://www.douglasmotorcycles.co.uk/ but they do not appear to be very active on the web.
I strongly recommend that you join which will give you the usual club things such as a magazine and technical advice but also access to factory records, (find out just when you bike was made, check matching numbers etc) transfers and of course a lot of spare parts new, new old and just old.

Money is not a problem if you send your credit card details and I have never had any concerns with them, all sorts of bits and pieces including gasket sets, bearings, valves etc arrived in short order and for reasonable prices.