It can be found in the Wise Woman's Yurt in the Urshilaku Camp. It is owned by Nibani Maesa, and can only be acquired by stealing it. However, if the Master Index plug-in is installed, she will sell it for 25 .
Talk to Folms Mirel about the topic, 'work'. He will request that the Nerevarine find all 10 propylon indices and bring them to him.