The first and only time I have seen 'Tango' at a retail nursery was at Bonita Creek in San Diego in June 2008. UCR made an "early" release of budwood in June 2007, limiting 12 buds each to commercial nurseries. CCPP - Citrus News
I thought at one time I read that Four Winds was going to have 'Tango' available in 2009, but I have not seen it listed yet. Since they make mail orders and are in northern California (away from quarantine areas), this would be convenient for many hobby growers.
Edit: The trees that were at Bonita Creek were grown by the wholesale nursery Brokaw Nursery so other retail nurseries buying stock from them may also have 'Tango'.