Reference works
dictionaries, encyclopedias, writers' reference handbooks, books of lists, almanacs, thesauruses, books of quotations, and so on
Internet
dictionaries, encyclopedias, writers' reference handbooks, books of lists, almanacs, thesauruses, books of quotations, etc can be found online. In addition, writers can use search engines or portals (sites that list many resources and websites) to gather ideas and information.
Student-created sources
For example, a student's personal dictionary of words to know or spell, note cards, graphic organizers, oral histories, and journals.
Other sources
Film, art, media, and so on.