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Polonium 210 is available commercially in the US without a license. I don't know whether it is on the NPT list. Probably not. You don't need to control a nuclear reactor in order to lay hands on this stuff. Take a look at these anti-static brushes, e.g.:
www.optcorp.com/product.aspx?pid=1818
I don't think these guys have a nuclear reactor lying around. Polonium 210 has a number of commercial applications, and is also used for many kinds of scientific research.
We may have abundant cause to think that the Russians were involved in this incident. But, the notion that a state actor is required to explain the presence of polonium is simply wrong.