Newspaper reports suggest it is hard to come by and traceable - not so sure, the dose might have been as small as a millicurie or two, and you could buy that for about half a million dollars - it is $690 per microcurie retail.
This from Steinn Sigurðsson who would seem to know what he's talking about. At first glance I thought -- hey, maybe you shouldn't be able to buy this stuff from a website that proudly proclaims "No NRC license required! All our radioactive isotopes are legal to purchase & own by the general public." But then again, sure you could kill someone by buying $500 grand worth of Polonium 210 and using it as a poison but why would you? Put this together with the Yuschenko incident and it certainly looks like someone in Russian intelligence circles really enjoys showing off their ability to find obscure ways of trying to kill people.
I have very little to add to your MSM coverage of this story except to note that I've seen Boris Berezovsky's name tossed around in relation to this in a manner that seems to imply he's some sort of heroic dissident rather than, say, a leading member of the circle of horribly corrupt gangsters who robbed Russia blind during the Yeltsin years and just happen to have been run out of town by the new gang.