There are numerous online libraries of sound effects, including FreeSound.org, FlashKit.com and FreeSoundEffects.com. As of 2015, all of these websites allow users to download sound effects as MP3 files. FlashKit.com has an extensive collection of sounds available as MP3 files, all of which are completely free. FreeSound.org is also completely free, but some of the files are only available in the WAV file format. FreeSoundEffect.com has both premium and free sounds.
To find a sound effect, visit any of these three websites and use the search box at the top of the page. You can also browse sounds on FreeSound.org based on tags, packs or even at random. On FlashKit.com and FreeSoundEffect.com, the sounds are organized into categories. FlashKit.com also allows users to search specifically for looping sound effects.
Most of these services have fairly extensive libraries that contain different sounds for personal or business use. Some of the websites offer forums where members can discuss relevant topics. Many of the websites also have rating systems and comment sections for uploaded files. The quality of the files may vary since most of the sounds are created by members of the websites.
Some sounds are only available for non-commercial usage or may have other restrictions, so check the license information before using a file.