That is a hard question, and the simple answer is I don't know.
I guess the best answer is life--I see things, hear things, think about things that turn into thoughts, concepts, arguments and ideas and a write about them. I have some favorite authors, John Steinbeck, Mark Twain, Harper Lee and a few others, but I do not follow their style, and I have not written a book. I go for the short story, the essay, the column or the letter.
To give the philosophical answer, I guess I would say my inspiration comes from everywhere. I see a story in a newspaper. I read a Hub. I hear a conversation. Some thought just comes to mind, and that turns into something written. Some are still private, not sure who I was writing to or for. Others works are published for here, some in another no-pay but great exposure publication and a couple of paying jobs--nothing to get excited about.
There are tools I have developed, such as listening and remembering past events that affect my writing, but defining the source of inspiration, will probably remain an elusive task for a very long time.