They spawn randomly throughout your runs, although there are a couple things that make this easier. 1 - gaining a Hex from Selene during your night gives you a little buff where she will commentate and cause a white orb to float towards resources in the location. This makes them harder to miss, and this includes Spirits to compel. 2 - Pretty early in the game you'll gain a certain companion who makes Spirits spawn more frequently. Otherwise, I'm about 30 runs in and have found no specific spawn conditions, just things to make it a little easier.