It's not that difficult to get. Better question is why it was used - as far as I understand using polonium for that purpose is impractical, expensive and dangerous for perpetrator. The only reason I can anticipate, is that the purpose was achieving as much media effect as possible.But Russian maffia or Chechens have not Polonio at hand, havent they?
May be. There were wanted Russian criminals in Britain at that time - oligarch Berezovsky, former Chechen authority Zakayev, etc. Why kill harmless person from all possible others?Maybe Putin simply ordered "kill that SOB"
If he possessed information which Putin didn't want to be disclosed, then why they gave him poison which requires several weeks to kill person?