Give it shot and try out this place Continental Traders. The no. is 4314734. His got brands of all sorts and can help you tune up or down your music instruments.

The shop is located near Souq Al Jaber - near Standard Chartered Bank.

Plus he's got a store which if you find difficult to go to the souq - you could go to Al Mansoura. Hopefully it is more convinent.

Good Luck!!