A local science or children's museum may sell simple lenses, prisms, mirrors, etc. Laser pointers are now quite inexpensive and can be used for optical experiments with colored filters, lenses, and optical fibers. You can buy laser pointers at office supply and electronics stores.
Find optic component and laser kits for all ages. Features laser pointers, magnifiers, prisms, unique lighting kits, and numerous other optics-related projects.
Science Kit & Boreal Laboratories
Science Kit & Boreal Laboratories is a leading provider of science education products to K-12 teachers, home-schoolers and science hobbyists throughout the United States. Sells prisms, mirrors, and lenses.
View their online catalog for their extensive selection of optics and optical components. They offer experimental and commercial grade optics such as lenses, mirrors, prisms, magnifiers, lasers, and much more. Also see their digital catalogs on optical topics.