Most of the materials for the projects can be found around the house, or are just scraps in workshops. Visit small businesses working with the materials you need and you will be surprised how welcome most tradespeople will make you feel when you show them what you are trying to make. When you go accompanied by and adult, ask if you can explain the project yourself, it will give you a “can do” feeling and while you should accept a “No”, most of the times you will get a “Yes” and you walk away with enough scraps for several projects.
Also check the recycling box, there is lots of good stuff in there to make interesting projects.
This is not a complete sample of materials you need, but most materials are common or can be bought for not too much money, and they will last for several projects. Just collect scraps of wood, plastic, tin cans, elastic, some glue, and so forth and you can build up a surprising cache of goodies that will serve you well for many of the projects listed in the side bar above.