You can source sound effects and music from several online sites.
Youtube
If you sign up and register for a Youtube account you can use their Audio Library to download free music and sound effects. This can be found in the Creator Studio.
FreeSound
Freesound is a collaborative database of Creative Commons Licensed sounds. Browse, download and share sounds.
Sound Bible
Sound Bible provides free sound effects licensed under Creative Commons or released as public domain; N.B. some effects are licensed for personal use. While the search tools are limited and their catalogue lacks the breadth and depth of commercial libraries, you may still find the effect you are looking for.
Free SFX
Free SFX has about 8,000 free sound effects. While the search tools are limited, everything is categorised helpfully
Getty Images
These images are not free but are of high quality. Prices seem to be around £7 for each effect.
Videvo
Videvo offers free stock videos and motion graphics for use in any project. You may use these video clips free of charge, in both personal and commercial productions. Video clips that carry the Creative Commons 3.0 license must be attributed to the original author.