Asbojohn,
You could also try Vinci autoroute services here:
http://www.vinci-autoroutes.com/fr/conditions-circulation which is where I got mine from.
It works on all the autoroutes and also in >90% of the secure Vinci car parks.
They are very specific about where you need to put the transponder on the windscreen (high up, centrally behind the mirror, in an area not covered by heat reflecting foil or heated windscreen wires) but in practice we've found it isn't critical at all. I don't fix mine to the window and my wife just holds it up as we approach the barrier. You can see the detectors, usually either above and to the left (i.e. drivers side) or centrally above the car as you approach the barriers.
In the Mog, we're hopefully going to hold it just above the windscreen (if we've got the top down) or out of the side screen window if not.
It really does make toll roads much easier, and it's great to see the French expressions as you drive through without stopping... especially when they have assumed you are in the wrong lane.
P.S. I've just realised this thread was started in 2012. It took me a while to workout why it wasn't in the new posts list on the left of the screen
