Desk fan in Paris

You should be able to find one at the following stores:
Boulanger (not to be confused with boulangerie)
Darty
FNAC (yes, FNAC recently bought Darty but the two still are operating as separate enterprises at the retail level)
Hypermarchés such as Carrefour, Casino Géant, Auchan, E. Leclerc
Homestores such as Conformama
Large home improvement stores such as Leroy Merlin or Castorama
A better-stocked droguerie (a corner hardware and home store, generally located in cities) may have them too, but you'll pay likely more
BHV Marais, which has a large store in Paris, may carry them, but I'm not sure.
Some Muji stores (a Japanese home and clothing products store) have small USB-powered fans (some Mujis are just clothing though -- be aware of that).

These generally would not be dual-voltage travel fans, but regular table fans (ventilateurs) or USB fans.

Everywhere k has mentioned, but the surest and easiest in Paris is probably the BHV, in the basement. Metro Hotel de Ville, across the street from city hall. Second in Paris would be a Darty, branches scattered around the city.

LOL - I saw the title and I thought this was an inquiry of someone who is a fan of fancy Parisian desks looking for a museum dedicated to such. And I bet there is one if you look hard enough.