I don't get asked this questions as often as published writers, probably because I don't discuss my writing activities often with non-writer friends, nor get interviewed about my work. (Upon reflection, it's odd that journalists would ask where ideas come from. As writers themselves, you'd think they'd know. Perhaps this query comes more from talking-head TV interviewers, who don't actually deal with ideas for their jobs.) I've been tempted to reply that all ideas are issued by the Writers Guild (a problem now, with the strike and all), or that there's a sweatshop of Bangladeshi children who crank out ideas for 17¢ a day. I'm afraid that people might think I'm telling the truth.
In case you're not a writer, I'll let you in on a secret: Getting ideas is not the hard part. I have lots of them. Years ago, I split my "Ideas" file on the computer into smaller files by category, to better organize the multitude. I fill the small notebooks I always carry with ideas that just come to me. Which is where I get my ideas: They just come to me. Sometimes they arrive during conversations, or when I'm busy with a task that leaves my brain relatively free (e.g. driving, washing dishes, showering). Some great ideas happen when I misread or mishear something, and the resulting combination of concepts triggers something I wouldn't have come up with on my own. Sometimes, although this seems rare, ideas happen when I'm actually trying to think up new ideas.
To me, asking where I get ideas is like asking a potter where he gets his clay. Raw ideas are cheap and plentiful (Sure, coming up with ideas is like any skill: Some people have more natural talent, but anyone can improve their ability with practice). It's more like raw ideas randomly drop from my head; imagine that they look like dirt clods. Most of them get tossed into one of the sacks I have, to be revisited later. Sometimes I notice a gleam under the dirt, and I'll wipe the dreck off the idea. If I like how it looks, I'll carry it around in my pocket, taking it out to admire it, or make it cleaner and shinier. If it shapes up well, and there aren't any serious internal flaws, I'll develop it. I cut, reshape and polish it until it's pretty enough to show other people.
So, if you want to impress an artist, don't ask where they get ideas. Ask how they select which ideas to pursue. Or how they transform the raw material into objects of beauty. Now those are interesting questions...