Even if you could find individual sticks, you will likely find that you will pay as much for the ones you need as you would for the whole set.
Also, don't be so quick to give away the odd colors. They have their uses. A little bit of bright powders can add texture and character to trees, shrubbery, rocks, earth, roads, roofs and much more. Don't overdo it, but a bit of subtle contrast can really make a scene pop.
