Go to Boulevard Beaumarchais. It is a street in Paris (3rd arrondissement) where you can find a lot of shops that sell photographic equipment (secondhand and new). Here you can find a few addresses (but there are other places on the Boulevard). Sorry, it is all in French.
As for the prices I don't think there will be a huge difference between France and UK. Check the prices before you leave the UK to compare them to what you will find in Paris.