I usually just use the Microsoft clipart--I figure, I own copies of Word and Excel (not to mention Windows), so I've paid for it already, so why not use it?
I use the microsoft stuff or troll flickr for appropriately CC licensed stuff .. but most of my stuff these days is for edu-type presentations so i have a credit page at the end.
aah. thank you delicious. try http://www.openclipart.org/