I'm looking for a guitar shop that has string and other supplies to fix and maintain guitars.
Best if the shop would be close to the city center or wenlinjie area.
Any ideas?

There are a few guitar shops that also fixing supplies on the north-west corner of the Yieryi dajie/jianshe lu crossroad.

There are many music shops on the west side of Xichang lu between Jinbi lu/Xiba lu and Renmin/Chunhui lu. Also, the next street over, parallel to Xichang lu (to the west) there are many more. I'm sure someone there can direct you to a "fixit shop" Good luck.