Where did C.S. Lewis get his ideas for Narnia?

Generally speaking, they came from his imagination, although they reflected his Christian beliefs. He said in a letter that for him the books started with an image or picture in his mind, such as a faun carrying parcels in a snowstorm (The Lion, the Witch, and the Wardrobe). He would usually have 2 or 3 such images and then try to make a story that would connect them in some ways. Aslan came from him having dreams about lions while working on the story.