For the fastback, (and presumably series cars too) they are the same for the three plugs, part number 5220713. The original ones are folded copper with a small cutout to wind onto the thread of the drain plugs. Internal diameter 0.75", external diameter 1". If you are planning on using Dowty washers, you might need to increase the ID to clear the plug threads!
Could any of you tell me what size washer I require for the engine, gear and diff drain plugs? I'm going to get dowty washers and need the sizes so I can them ordered before I start work.