There are lots of sources of free sound effects on the internet. You could try searching on Google, or browse through some of the links below.
Right-click and "save target as" with a name you'll recognize in the folder you can find. Import the sound into Premiere, edit it if necessary, and put it on the timeline where you need it.
Sound Dogs:
Misc. Sounds (Sound Dogs)
Vocal Statements (Sound Dogs)
Royalty Free Music (Sound Dogs)
Samples and Loops (Sound Dogs)
WAV source page: http://www.wavsource.com/
http://www.externalharddrive.com/waves/waves.html
WAVS page: http://chataholic2.homestead.com/wavs.html
Looney Tunes Soundsource: http://www.nonstick.com/sounds/
