If you don't mind using a headset that is not correct for the Gitane, you can get a Stronglight A-9 from Harris Cyclery for $50 as I did.
http://sheldonbrown.com/harris/headsets.html#french It's much lighter than the original P-3 and has needle bearings. A P-3 is extremely difficult to find and will cost you at least $100. I think the A-9 is much better looking, too.
Sheldon says that it doesn't turn as freely as a ball and cone headset. That's true on the workstand, but I can't feel it when I'm riding.