In Marleybone there is a secret area in the Isle of Dogs sewers full of cat burglars. There is a store and a tavern. The outfit, hat, and boot vendors are in the store, the trinket, training tome and jewelery vendor are in the tavern, and the weapons vendor is behind the tavern. This is the best buy able gear I have found in Marleybone.
Thank you for the helpful gesture, but the part of the sewers you're referring to is simply Under Town. It's a part of Marleybone that everyone goes to eventually through the quest line. So yes, there are good stores there with good gear, but it's not exactly secret.