Generally, please do not use copyrighted material in any of your products. Material you can use includes the following:
Material published as “public domain.”
Material published under an appropriate Creative Commons license (there are several different licenses, so please always check what license applies).
Material which you buy online, and where you buy a license to use it (it can be quite cheap).
Material you make yourself.
So, make use of some of these sources of usable images, music, sound, etc.
Sound recordings, sound effects and other free audio content: http://www.freesound.org/
A great archive of BBC sound effects (read their RemArc licence first): http://bbcsfx.acropolis.org.uk/
Creative Commons licensed images: http://www.flickr.com/creativecommons/
Moby has made 150 tracks available for free to "anyone in need of free music for their independent, non-profit film, video, or short" - to access there is an online application system which takes around 24 hours: http://www.mobygratis.com/
Free patterns for graphics: http://subtlepatterns.com
Mostly free icons: http://thenounproject.com/
There are a number of free to use online tools that can help you in building a prototype.
Canva is a useful free to use online tool that helps make graphics and storyboards - https://www.canva.com/create/storyboards/
Piktochart is a free tool for making infographics https://piktochart.com
Adobe Spark is a very easy to use video making tool: https://spark.adobe.com/about/video
A fast and free app that allows building prototypes for apps on iPhone: http://popapp.in
Online tool for creating website wireframes: https://gomockingbird.com
Service design tools: http://www.servicedesigntoolkit.org/templates/
NESTA's excellent guide to prototyping http://www.nesta.org.uk/sites/default/files/prototyping_framework.pdf
Some service design methods with download templates from Open Change http://www.openchange.co.uk/how/
You can download a Handbook of Digital Storytelling - a resource which helps people develop digital storytelling skills. It is intended for use by anyone who has an interest in using digital media literacy skills for storytelling and contains themes and examples of work together with suggested exercises to help develop digital storytelling skills.
Here is also a post introducing to using digital storytelling as a service design tool.
Want to know more?
There's some great material available that goes into real depth about service design and public services. We suggest that you start with this reading list.