I've always thought that when writers say they don't know where their ideas come from, it's a bit of a cop-out. Ideas are everywhere; you write about the things you're obsessed with, or think are cool, or think other writers aren't doing right. It's not actually coming up with the idea that's hard, but executing it successfully.
To wit, here's David Moles' fascinating series of posts on coming up with ideas, in which he somehow fuses together Tom Waits and HP Lovecraft:
- A Change of Clothes: Sexuality, procreation, the human body, invertebrates, marine life in general, fat people, people of other races, race-mixing, slums, percussion instruments…
- Digression: the trick: Myths of the 20th century
- The Names of Towns: …caves, cellars, old age, great expanses of time, monumental architecture, non-Euclidean geometry, deserts, oceans, rats…
- Digression: the trick (pt. 2): Lateral thinking
- Something to Eat: …the New England countryside, New York City, fungi and molds, viscous substances, medical experiments, dreams, brittle textures, gelatinous textures…
- Some Weather: …the color gray, plant life of diverse sorts, memory lapses, old books, heredity, mists, gases, whistling, whispering…
- Digression: the trick (pt. 3): “You have to make them out of things you’ve found”