I've asked that question of myself often. Once I purchased the book Secrets of Magical Craftsmanship by Salvador Dali. Now THERE'S a guy with lots of ideas who is ready to spill his secrets. His secret?
As you sit in your stuffed chair, have your valet place your keys in your hand and a dish on the floor. Sleep. When the keys hit the dish, jump up and paint feverishly for an hour from what ever was in your dream.
Okay, I handed my keys to Deborah, but was told, "I'm not your valet!" A setback, but I persevered on my own.
An hour later, the keys missed the dish but I jumped up and started squeezing clay feverishly.
Not even close to an idea. Just lumpy clay.
It dawned on me that Dali has his path to ideas and so I must have mine. Okay, Harold, answer: What's your path?
For me, ideas come from everything I have ever learned or done. While that sounds big, it actually limits me to only what I know. Eastern thought, African rhythms and bison instinct never play into my ideas. So I go with what I know.
Then I combine from those personal physical and spiritual ideas to draw the essence of a new idea--new to the world--a message in an idea, a metaphor. It helps that I have self-diagnosed ADD. That lets me move through candidate ideas quickly. It also helps that I can comfortably picture things in three-space. I don't know if that can be learned, but happily, I've got it. I look to get a dozen or so ideas at a time.
Filtering comes next. Anything not joyful, thought provoking, attention grabbing or truthful, goes away immediately. Next go the impossible ideas; flames as sculpture, weightless bronze, and live figures.
Then I actually take a page from Dali--sort of. I sleep on an idea.
Those of us of a certain age remember when hotels and rail-car sleepers included a small two-door box built between the room and the hall, one door to the room and one to the hall. Overnight, you placed your shoes in the box and the next morning they were cleaned and shined! Magic! Sort of.
I have a personal shoe-box for my current idea. I think about my idea and go to sleep. The next morning, magically, the idea has been cleaned and sometimes expanded, problems have been solved, opportunities pointed out, connections clarified.
No dreams seem to be involved, just wide-awake looking into last night's shoe-box to see what magic has happened.
Works for me, anyway.