Apart from the Elder Scrolls bounty for defeating Skolas all other exotic bounties are dropped randomly upon completion of daily bounties. Basically try to do as many bounties as possible on a daily basis and eventually the exotic bounty will drop. Just need to be patient. There is some rumour that exotic bounties drop more frequently if you spam the bounty bot or Eris Morn with a set of completed bounties (i.e. 4 or 5 at a time). You will usually be given a choice of three exotic bounties. If you want the Thorn opt for the bounty titled 'a light in the dark'.