Hello apoomoopa,
You'll find the content you're looking for in a couple of different places. Starting from the Macintosh HD (or root level of your boot volume), in Library/Audio/Apple Loops, and Library/Application Support/GarageBand/Instrument Library. Once you've located these folders, you can highlight the folders contained within, and choose 'Get Info' from the File menu to determine the best course of action to free up space. If and when this content is removed, it can always be redownloaded.
You can find this option by choosing Download More Sounds from the GarageBand menu. If you previously purchased this content, select Restore Purchase to install the content.
GarageBand, Logic: Where loops get installed
http://support.apple.com/kb/HT1045
Logic 8: Moving Jam Pack Instrument Library to external HD
https://discussions.apple.com/thread/1829509
Expanding GarageBand for Mac (v10.0) content with an in-app purchase
http://support.apple.com/kb/HT6031
Cheers,
Allen