This is a very important product: I would advise everyone travelling to or through France should obtain some of these in advance from suppliers: make sure they are equipped with these and all the other necessities such as fluoro tabards, warning triangles and first aid kits, to be compliant with the law and to be able to placate the local police operatives. Be aware though that the new law about breathalyzers does not take effect straight away, there will be plentiful supplies available when you arrive in the country, at supermarkets etc. The law apparently comes in on 1 July 2012, but will not be enforced until 1 Nov. 2011.
[UPDATE.] I have recently come back from a fortnight in France, word on the streets is that the fuzz can stop you, sniff, ask to see your Alcopop device and can warn you if you don't hsve any but apparently there is still no legal procedure yet for them to do you for not having them so they are not bothering any more than that ... (maybe by November someone said). Oh, and don't worry about prices, they are still quite expensive in shops etc in Uk, and of course on certain 'transport providers'and 'motoring associations' (you know who you are) but if you can get to a supermarché you should pick a pack up 'cheap' (we saw some in Leader Price for under 2 euros).