Paula,
The Tango has been available in retail nurseries for a few years now. Let us know where you are located and perhaps a member here can help you pinpoint a nursery that has them in stock.
By the way the Tango is not a mandarin. It's parent is the tangor (orange cross mandarin) W. Murcott. Self-pollinated flowers of W. Murcott were irradiated in hopes of producing a seedless variety. Some of the resulting sprouts did develop seedless fruiting trees, and what was deemed the best of these became what we now call Tango. You can read more about it here:
Home Fruit Production - Mandarins .