Radiance (my favorite) if you are in central Midtown, Argo, or even Whole Foods for a little farther north, but not much else if you are looking for a good selection of loose leaf teas. But, if you are willing to venture a little further south west, an old shop with NY attitude awaits.
Empire Coffee & Tea, a great store that sells a nice variety of loose teas.
The store was supposedly established in the early 1900s (I'm not sure if it's always been at that location), but this is not one of those quaint West Village shops with creaky floors that gently sag into the pavement. No, this is a utilitarian store front, with bags of coffee everywhere and containers of tea in the back. A few teapots and coffee mugs can be found, but you're here for your leaves and beans. There is also a to-go counter where you will find coffee and teas by the cup, and a few benches if you need to sit while you sip.