You can always order them from any butcher.  Halles de Rives are very good, as are Boucherie du Mollard and the small butcher opposite Inglewood near Lady Godivas is exceptional.  They're not particularly cheap from these places however.  Aligro also has them, will be cheaper, they have frozen too.

Plenty of places in France also, try the larger supermarkets or any butchers in towns.   I'm sure someone here can give better advice re French places.

Like Royle said, you will see at all French supermarkets to have fresh turkeys around the first week of december up until Xmas. Swiss supermarkets are about 2 weeks before.

I also will get a turkey for a dinner party of 15 persons, but I am not sure how big a bird I will then need for 15persons. Anyone know?

Turkeys are not cheep, ad 2.5kilo bird cost about EUR 15-20.

Finally, I recall buying a turkey from the Coop Blandonnet for thanksgiving which is always at end of November, I had to first ask to check if they would have some, they seem to cater more and more to the expats :)

Anyone know where you can buy at a 'reasonable price' a traditional English christmas pudding? I need also for the dinner, as most attending are English and they all love it.

Jim's I'm sure.  Cant guarantee a 'reasonable' price though.

I bought a frozen turkey from Leclerc supermarket (just off the roundabout after Mategnin Douane, Meyrin).  I would think a 12kg turkey will feed 15. If you buy frozen, it will need a couple of days to defrost and cooking time will be in the region of 8 -9 hours. You can buy parsnips (a.k.a. panais) at the Migros supermarket. Check import allowances on poultry.