You steal them, of course!
Ronald Burt's theory of Where Ideas Come From explains:
Got a good idea? Now think for a moment where you got it. A sudden spark of inspiration? A memory? A dream? Most likely, says Ronald S. Burt, a sociologist at the University of Chicago, it came from someone else who hadn't realized how to use it. "The usual image of creativity is that it's some sort of genetic gift, some heroic act," Mr. Burt said. "But creativity is an import-export game. It's not a creation game."
Mr. Burt has spent most of his career studying how creative, competitive people relate to the rest of the world, and how ideas move from place to place. Often the value of a good idea, he has found, is not in its origin but in its delivery.
[via the excellent Scamp]
This chimes with Mr Heath's new research that suggests that what communication says is insignificant relative to how it is said.
As much as I like theories that support my titular proposition, the way this theory is formulated actually represents an infinite recursion - if you get an idea from "someone else who hadn't realized how to use it", the question then becomes where did that person get the idea, and so on.
Ronald's line of enquiry stretches back at least as far as Locke, who wanted to understand where ideas come from [although he had a broader definition of idea in mind - essentially the object of any thought]. His answer was from sensation and reflection. Simple ideas are experienced directly by the senses - horse for example - and then we use our mental facility [reflection] to combine them to build more complex, abstract ideas - take a horn and a horse and you've got a unicorn, even though you have never encounted one in real life.
Early Western travellers to India thought that the rhino was a unicorn, because Indian rhinos have one horn, which is possibly why its zoological name is Rhinoceros unicornis.
Perhaps the most famous artwork featuring a rhino is this drawing by Durer, who had never seen one when he drew it, which probably supports the proposition that it's the delivery that matters not the root of the idea, but I suspect I've already digressed too far to reign this post back in so I'll leave it there.