Hi Archie
Check out Sounddogs at -
http://www.sounddogs.com/start.aspThey have tens of thousands of sound clips in either Windows (wma) or mac format. Just go to the search field and type in your topic (eg - smack, slice, knife, rip etc) and then click on any of the files it brings up to hear it.
Most generic words will bring up a couple of hundred sound files (plus lots of obscure music snippets as well for some reason), just keep clicking and listening until you get what you like.
Save your file(s), convert from wma to wav (Google for a free converter) and there you are. I usually then cut the file down to size using Sound Recorder (it's simple and its free with Windows !) then save the result in to a folder reserved for polished sounds. Then anytime I need a sound I just pick and choose from my own library.
Don't forget to name your files as accurately as you can (eg heavyBang1.wav, shortRip2.wav etc).
If you have anything in particular in mind let me know and I'll shoot you across one or two of my best samples.
Cheers
Roni