Sources for accurate period parts

Just curious as to where member go for those little bits and pieces - do people like Jonathon... arggh, what's his name? - stock them? Or is it a case of tapping up people who've parted out old machines? I'd rather not do the same myself as I'm already running out of shed, and I have another mower arriving in November!

I'm particularly thinking of things like washers and sprung washers as I have some missing from my Standard and the modern ones are a different profile.

I'm also missing the lid of the oiler for, I think, the clutch - so if anyone can point me at a likely source it would be hugely appreciated!