Inspiration can come from anywhere and I believe we’ve already touched on that subject. But inspiration without imagination is like a vehicle without a driver: it can get there, but something has to take the controls.
Just as there are an infinite number of sources for inspiration, there are also an infinite number of ways a person can build their imagination. For me it was playing with LEGOs and GI Joes as a kid. For other people it can come from playing cowboys and Indians with their friends. Many people simply get an awesome imagination from reading.
That is the important thing about imagination; knowledge. You need knowledge in order to do anything well and using your imagination is no exception. Reading is by far the best way to stretch your imagination. Reading different things can broaden your horizons immensely. There are so many different authors out there with so many different ways of looking at things you would be hard pressed to not learn at least one thing from any of them. Even if they aren’t particularly good, they can still teach you something.
In the end, imagination is something everyone should use, even if it isn’t write. Everyone has something they want to escape from and your imagination is the best way to do that.