Here are some Audio Resources that you can use with Garageband and other projects:

Soundsnap - free sounds and loops to download.  Use the mp3 files - they will work with Garageband.  (Do not click on the Google Ads to find more sounds.)

Findsounds - good source of sound effects.  Note that the wav sound format will need to be converted before you can use it in Garageband

Vozme - type in text and transform it to an audio file that you can download (control/click on the file to select the location to download the file)