Do you know where you’ll be staying in Paris?
If you’re able to carry all your gear and a boxed bike short distances, the B line Metro goes to CDG.
I’ve only ever searched for a bike box once in Paris close to the Montmatre area. There were a fair number of small shops. They cut up the boxes immediately to save on shop space. One shop told me they got bike deliveries early in the morning. Check opening times as shops in France may be closed on mondays.
Maybe send some emails out a week or so before you get to Paris to shops close to where you’re staying.
I’d advise staying an extra night to give yourself a whole day to find a box.
p.s my favourite thing to do in Paris is get up really early and wander the small streets as they are cleaned and the pidgeons rule!
When you find a bakery, fresh parisian baguette is the best in France and by extension, the whole world.