How To Construct/Purchase Inquisitor

Hi, I am playing as Maria Theresa, can someone tell me how I can purchase/build an inquisitor so as to defend my cities against those ghastly great prophets? Also, are there any other techniques to defend against them other than encircling/guiding them with military units?

You need to enhance your religion before you can buy an Inquisitor. Then you go to any city with your religion as dominant, go to the normal production menu and shift to buy as you would with any other unit, and then the Inquisitor should be available. It will cost you 200 Faith up to and including Medieval era, and more in subsequent eras.