Any writer you ask this will instantly reply with everywhere.
Articles and stories in newspapers, magazines, and on television often start a spark with me. I have gleaned information from obituaries, memoirs, and listening to people in restaurants.
In the movie, Young Adult starring Charlize Theron, she is a young adult novelist. And she is often seen taking bits and pieces of dialogue she overhears.
My professor, Susan Tekulve warned me that when talking with other writers you better quickly lay claim to anything you plan on using in a story - they might get it before you do.
In my current novel in progress, one of the ladies is a member of a 55 year old plus dance troupe that tours around the area performing at ball games, nursing homes, and for anyone who will have them. The idea came from a long ago news piece I heard one morning as I was getting ready. Dancing kept these ladies young. I filed it away in my brain for later use.
I also have my boxes, which are just photo boxes labelled with the working title. I have several ideas for novels that have popped up over the years and I tuck anything pertinent to that story - maps, articles, postcards, restaurant menus, dialogue, and bits of stories. I also have a file for each on my laptop.