Longy/Bard would be closest. It's on Follen Street. They used to have an extension division (this is what you want if you wish to take lessons from the professors but don't want to enroll in the degree program) but I don't know if they still do since Bard took over. It's a conservatory so if you don't want to pay the big bucks for a prof there's lots of grad students happy to make some extra cash by giving lessons.

There's also The New School of Music on Lowell Street nearby which is a community music school with private or group lessons for all ages.