For what it’s worth, second-hand info from
tricotin.com forum (post your question there if you
want more details):

The most mysterious and intriguing lead is that there
is a “créatrice de laines” rue Chativesle. No shop
outside apparently, you probably have to ring to get
in her “atelier”.

Then you have two yarn stores related to two of the
biggest French yarn mills:
* PHILDAR – 93 rue de Vesle – 51100 REIMS
* BOUTON D’OR (MERCERIE DES CHATILLONS) – place des
Argonautes – 51100 REIMS

And lastly you have big craft stores in the outskirts
of Reims, where they sell staple European brands such
as Katia:
* MONDIAL TISSUS – route de Louvois BP 4 – 51350
Cormontreuil
* PAS CHER – près du Boulingrin – Reims
* TEXTILOISIRS – rue Doc Robert Creusat – zone
commerciale CORA Nord – 51100 Reims