Be prepared to make the next costume when the time strikes!
This video is about what kinds of things to keep on hand. Sometimes they are remnants, others come from packaging - like rope handled shopping bags - before you throw the majority of it in the recycling bin.
Costumes can be expensive to sew in terms of all of the materials needed. That's why it's a good idea to save what comes through your hands. Evaluate everything!
It's amazing what you can do with things like that. But you must be ready when it comes time to sew . . .