The hotel Concorde Lafayette is situated at the 17th district of Paris, tube station Porte Maillot, line 1. Verify if you're staying there or at the Concorde Montparnasse, situated at the 14th district, tube station Gaîté (line 13)
Nearby the Concorde Lafayette hotel, there's Rome Instruments, situated at 54, rue de Rome, 8th district, tube station Europe, line 3.
http://www.concorde-lafayette.com/en/paris-tourism/directions.aspx
http://www.romeinstruments.com/
Nearby the Concorde Montparnasse hotel, there's HB Music, at 128, rue de Vaugirard, 6th district, tube station Montparnasse Bienvenue.
http://montparnasse.concorde-hotels.com/en/paris-tourism/destination.aspx?
http://www.hbmusic.fr/services_plan.php
Both shops have a large choice of guitars and ukuleles.
Otherwise, go to Musikia, the biggest music store in Paris, situated at 11, Boulevard Sébastopol, 1st district, tube station Châtelet Les Halles.
http://www.musikia.com/fr/pages/tout-savoir-sur-musikia/megastore-musikia
http://www.ratp.fr/en/
http://www.transilien.com/web/webdav/site/transilien/shared/documents/plans/pdf/plan_metro.pdf
http://plandeparis.info/plans-paris/paris.html