Stories:
https://storybird.com/
http://www.storiesfromtheweb.org/
Prompts and Story Starters:
http://www.canteach.ca/elementary/prompts.html
Other Online Resources:
https://www.commonsense.org/education/top-picks/elementary-school-writing-apps-and-websites
http://www.doe.virginia.gov/instruction/english/professional_development/institutes/2012/writing/elementary/phillips_abingdon_06.pdf