Think little notepads that are freely available in stores around town. You can pick them up at drugstores, convenience stores, grocery stores, dollar stores, just about anywhere. These are the perfect thing for artists like poets, writers, songwriters, scriptwriters, or even for everyday use to keep tabs on things for forgetful people.
They fit in a pocket and can be pulled out anywhere you are like in the car, on a bus, on an airplane, at work, or in school. All you need is a pen to jot down what it is you want to remember like poem fragments, a story idea, a shopping list, reminders, a to-do list, etc.
Limited because of their size, we can combine them with a larger journal that can be electronic or hand-written to expand, or develop the ideas you record. The crucial point is they are small and cheap and freely available in a multitude of stores.