A number of Web sites offer free craft ideas, and quite a few even pair them with lesson plans for you. Craft stores frequently offer free instruction sheets that you can adapt to preschool. Your local library should also have books on preschool crafts. Finally, use your wait-in-line time at the grocery to peruse magazines for craft ideas.
Disney has a wonder website full of great craft ideas (). The content is free and does not result in a bunch of unsolicited emails. Most are appropriate for this age or could be simplified to be made appropriate for preschool aged children. The crafts on this site also use materials that you would likely have around the house.