Hi Evan,
Regarding the Mk 3, the simplest route forward is to join the London Douglas Club, fees are at the moment £18-00 a year. Details can be found at
www.douglasmcc.co.uk the club information site.
The spares section carries a good range of post war parts including guides, valves, gaskets etc, we also attempt to stock the more commonly required pre war parts, sadly with such a large range of machines the stock is less comprehensive, however, we do make every effort to maintain the "consumable" items.
Dave