Hi Lonsdale73.
Yes, I have "a" length of 12.7 mm brass tube approx 12 inches long. The wall thickness is 0.25 mm, so if that's too thin for your job, I have another length exactly the same which is a tight sliding fit into the 1st piece. That also has a wall thickness of 0.25 mm also (so making a 0.5 mm wall - obviously!) and if that's not enough I have a 3rd length of 0.25 mm, ditto sliding fit into the 2nd, to bring total wall thickness to 0.75 mm (+ whatever slight extra thickness sweating it/them together with solder would make - I've done that trick several times and it does work OK, and - surprisingly - it does cause the OD of the finished tube to swell very slightly).
Where did I get it? As 2 posters have already suggested above, from the local model aero shop, and not hugely expensive I seem to remember.
You can have the 3 lengths (if needed) for nothing, BUT there's a rub (always is, isn't there?)!
You would need to repay the postage and while I've seen several gripes on here about the cost of UK postage charges recently, you lot ain't seen nothing until you've seen Swiss postal charges! I haven't checked, but at a guess (and I do have some past - bitter - experience) I'd say you'd be looking at at least the equivalent of 20 quid if not more, for tracked air mail.
Up to you, the offer is open, but at a guess I'd suggest you try local model aero shops yourself - some do mail order, and should be cheaper than Swiss Post.
If you don't know any local shops, try mags like "Radio Control Models" and/or "Flying Scale Models" for lists of shops all over the country (used to be "free for a gander" every month on the shelves at WH Smith, etc).
HTH