Only mentioned Cherbourg as we are going on a day trip next week!
We did the trip pourselves a few years ago and bought the licence in Rouen. Some friends want to get one before they actually set off, if that is possible.
If you search back you will get the web site address for the french canal authority. I suspect you can buy a pass on the internet. Two years ago I went into Calais but we moored overnight in the marina and walked round to the Canal authority office to buy the permit which works on a daily basis as I expect you know. You need to contact that office anyway to arrange for the sea lock into the Canal de Calais to open and for someone to go along it to open the bridges.