I have to agree with what others have been saying -- everywhere!
But I also keep a loose-leaf notebook. It started out being clips of quilts I would like to make or just enjoy looking at. But it changed over time and became a color inspiration book. I cut pictures out of magazines, catalogs, just about anywhere if the pictures feature color combinations I find pleasing. They mostly aren't pictures of quilts, but of fabrics, clothes, interior design, gardens and plants, just about anything that has an interesting color combination.
I refer to the notebook when thinking about a new quilt to get color inspiration. It's great to see the colors in an exotic plant leaf, for example, become the colors in a quilt. Inspiration is everywhere but I find it helpful to clip and save it when I see it so I have a record of the actual shades that struck me initially.