You shouldn't have to spend a lot of money to prepare an Instant Challenge. Most ICs use things that you can already find in your home or at a Dollar Store:
wooden craft sticks
You also shouldn't have to spend any more time preparing an IC than it should take for the kids to solve it. To save time, use stackable drawer inserts to store a variety of materials. That way, you can easily see what you have on hand and be able to quickly grab what you need. In most cases, it is absolutely fine (and fun) to substitute materials.