How does Bradbury get his ideas to write books? I'm sure he starts with a theme, but where does he go from there?

Download the videos of Ray Bradbury Discussing how he creates....exactly what you asked.
Look for 'postings', denoting Bradbury videos, follow instructions...

Another good source -- in addition to those -- is his book, "Zen in the Art of Writing".

He has all kinds of anecdotes about how stories and story ideas formed in his mind. For example, the story "Skeleton" came to him when he was sick and was suddenly aware of his adam's apple. This idea of the self awareness of certain body parts developed into a guy who could no longer live with his skeleton. He couldn't handle all those bones. He ends up having them all removed.

"The Lake" a story he repeatedly cites as his first good story, grew from a personal experience with death and loss. This all welled up in him and he wrote the story in 2 hours.

In the same way, his books often grew from short stories.

Anyway, if you are interested in Ray Bradbury, I was able to order mine (paperback and reasonably priced) on Amazon.com.

"Zen in the Art of Writing" Ray Bradbury. Joshua Odell Editions, Santa Barbara, CA. 1996.

Check it out.

Oh, another source is that he has all kinds of forewards, introductions and afterwards scattered throughout various editions of his works. In many of those he addresses the idea of creativity and idea development.

thanx