When I had a Series 3 109 sixpot (November 1973) I was told the ashtray was very rare. I don't smoke but I used it for little screws I found lying on the floor. So, you may not find ashtrays, is what I'm saying.
A friend here had a 1975 "Basic" 3-door sixpot 109 at the same time I had mine, and we ordered brake lines together from Automech. They informed us that there were 3 or 4 different brake configurations on the S3 sixpots and they needed precise information from us about the size of each fixing, to ensure it would fit. His fixings were indeed different from mine. Something to be aware of.
Luckily I didn't have to fix much on that S3.
Based on my 2A, though:
The carpets and floor mats, if you have a Station Wagon.
The brake drums, though S3 drums with 16mm studs are more commonly found than S2A drums with the smaller 9/16" stud holes.
You can rebuild a sixpot water pump easily, using a kit, which is ubiquitous. I've done two: mine and a spare.
The rear fuel tank issue on my 2A is irritating -- the replacement being a bit smaller than original and only compatible with the bayonet sender -- I'll have to put more effort into reviving my original tank.
The speedo cable is expensive and hard to find for 2A sixpots. I don't know if this supply is an issue with S3's (the cable is different, of course). I think there's a different part # for left- and righthand drive for S3, since the speedo is on the driver's side and RHD is a much longer route. Just get the longest you can find.
The propshafts can be hard to come by and costly.
Engine parts are available at John Wearing and other Rover suppliers. They cost more than 2.25L parts but less than parts for modern cars. There's no engine part, yet, that I haven't located, somehow.
Ah, yes, the exhaust manifold. Its weight in gold if you can find one.
If you have an 11AC alternator with the external regulator, I found that those were also used on Jaguars, so look for spares in that circle. My current alternator has a sticker on it that says it's for an E-type! You can get replacement diodes if yours blow. When my original bimetal voltage regulator failed, I got a solid state one at an autojumble that looks identical to the original on the outside. (from an early 70s Jaguar). Later Series 3's may have different alternators than the 2A and the early 3's?
I got a like-for-like britpart gearstick recently from paddock, exactly like the original (LHD) so at least for 2A's there's no problem, there.
If you're really in a desperate spot, let me know, I have a good spares stash and access to more, as sixpots were common, here. I'm not likely to ever need most of the things I've got, judging by how little I've driven in the last 3 years.
The shipping and customs from Switzerland could be enormous ... come for a visit sometime?
Well, that's a start anyway
Jeremy