Germany is a good place to find an airhead GS, although they're as cheap as they used to be for some reason. Second hand prices went up with the introduction of the Euro, as did most things.
A good place to look is TourenFahrer magazine. It's out monthly & normally has at least 10 GS' for sale. My girlfriend is German & gets TF every month.
If you have trouble understanding the adverts in TF, let me know & I'll provide the english translation's of the common bike parts & abbreviations that they use.
Please note that quite a few of the German bikes have covered a lot of km's but can also come equipped with some worthwhile accessories.
There are few for sale in the UK. I'm selling my 1993 R80GS Paralever sometime soon, it's the wrong time of year at the moment.
BTW, wherabouts in France are you?