Yesterday, Barbara, stole my thunder. Where do writers get ideas? They hang in the air. Really, if your brain is geared for "what if" then you see story ideas everywhere. You read the paper and there's an idea. You miss you 20th high school reunion, there's a story idea (which I did. I got ready to go the weekend after the reunion, but what a great for a story. Hero and Heroine make the same mistake.)
I will say, ideas fly at you. Now writing those ideas and plotting them is a whole other animal. And that idea might undergo some changes and morphs into another thing, but the ideas are just waiting. I had a book come from a teasing session with my husband one afternoon in the kitchen. We were throwing around titles and he came up with "How Tall is Red?" I liked that title so much, I did a book around it. I thought of following that book up with the other titles, "How far is Yellow", "How Deep is Blue", "How Wide is Orange." You get the idea. How Tall is Red did get published, but the name was changed to Undercover Husband. I still used the line "How tall is red?" The heroine got a note from her dead husband with the book title on it.