Where is the best place in Cincinnati to buy music equipment?

That depends on what you're looking for.

When my kids needed basic instruments (viola and saxaphone) I got them at Buddy Rogers. They still have several stores in Cincinnati that sell or lease instruments to lots of school children in the area. They repair them as well: http://www.buddyrogers.com/

The Baroque Violin Shop sells, repairs and leases fine stringed instruments out of a shop in a beautiful old home on North Bend Rd in College Hill. I was there a few times with my daughter years ago, and I remember the owner as professional and knowledgeable. http://www.baroqueviolinshop.com/

Jules Azzizi Violins is on Elm St. near Music Hall: The owner relocated his shop from New York City because of Cincinnati's " rich musical heritage to be rivaled by few US cities." What a compliment! The shop makes, sells and restores stringed instruments. http://azzi-violins.com/