Paul Graham, well known entrepreneur and business writer, has
published a very nice essay on where, and how, startups get their ideas.
What people usually say is not that they can't think of
ideas, but that they don't have any. That's not quite the same thing.
It could be the reason they don't have any is that they haven't tried
to generate them.
He goes into some detail about ways you can generate ideas and how
to tell a good idea from a bad one. And a whole lot more. It's a bit
long, but a good read even if you're not starting your own business.