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Yes, you can buy Polonium 210 readily on the internet. Here's one site who sells it for $69:
http://www.unitednuclear.com/isotopes.htm
The thing is, this is a tiny amount, used for testing radioactivity meters. By their estimates, you need to buy 15,000 of their kits to have a good chance of poisoning someone. And it would be technically very difficult to extract the polonium from 15,000 electroplated needles, and recombine it into a whole.
The only people who had the technological skill to pull off this poisoning are to be found among the nuclear powers, and probably only one or two of them at that.