Ideas can come from anywhere... A storyteller or idea person must have their radar on and be open minded at all times. Having a notepad or sketchbook on your person is such a good idea. (I have a sketchbook and a notepad at work, a mini sketchpad in my bag at all times in case the opportunity to do some gesture drawing arises or an idea pops up and sketchbooks and oodles of paper at home.) I have recently regimented the practice of having available paper at all times in my daily life and I'm trying to keep my mind open.
Something that I don't always remember to do but when I remember I do - is not only write down my ideas - but how I thought of the idea! It's interesting once your mind starts churning and the train of though can bring out more and enrich existing ideas futher. I am sometimes confused at myself when I come up with an idea that I think is compelling but for the life of me I can't remember how I thought of it!
Ideas can come from life experiences, daily life, observation, listening to people or podcasts, the radio, reading the paper, reading a book, poetry, from looking at magazines, photos, fashion shows, art shows, museums, websites, blogs, from memories, hearing a single word, while sketching, from nature, food and objects, watching animals and children, while watching TV or a movie... It's not good to copy ideas but to be inspired and let events/people/places/etc. fuel your train of thought.
Write those ideas down! I like to take a bit of paper to my bedroom at night if my brain has been going, oftentimes a lot of ideas and visions come to me as I'm falling asleep and my mind is on fire. I also write down every single idea. I remember in elementry school being taught the idea of 'tree brainstorming' and to write down everything. It gets information out of your mind and can jog ideas later. A 'tree brainstorming chart' is where you are writing ideas down and other ideas 'stem' from that.
It's also a good idea to be organised. Sometimes I write down a thousand things, all crammed quickly and messily onto a single sheet, that way when I glance over it later it's hard to focus on any one thing or even read my own writing. Organisation and clarity are very good for the artist.