I got some from amazon (http://www.amazon.co.uk/gp/product/B001P5SPJW/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1), these come with instructions. Luckily for me I did not need them as it was daylight at 7pm, so they stayed tucked away in the glove box.
I also got a GB sticker for the number plate , but i did see quite a few cars without GB stickers (better to be safe then caught out by French Police)