We bought ours some time ago via Amazon, although they have only just arrived. About £10 for a pack of 5 I think. We didn't want to find ourselves stopped by an officious plod just as we left the port and then having to explain in poor French that we didn't have to carry them yet.
I understood that they were both for self-use and also for use by the police if you are stopped - hence the need for 2. Apparently it's a savings thing!
I have also read somewhere in the past week or so (it could have been on this forum) that the police are focussing on foreign (for that, read British) drivers leaving the northern ports for spot checks. If they've bothered to stop you for one thing, such as breathalysers, they will also check everything else, in my view - GB sticker; fluorescent jackets; headlight converters; red triangle etc. etc. That could be interesting - have you ever noticed how many people don't have a GB sticker or number plate and also how many have lights, number plate and so on obscured by bikes on the back?
Jo