If you wanna go old skool (like me, for now...) you can use webdings etc.
Or you can google images - use key words like 'icon' etc. It's a pain as it's a bit hit and miss for what you'll find. What I'm planning on doing soon is making my own set of coloured icons.
Failing that, you can always ask other skinners if you can borrow their images - most of them stole them off the net anyway....
I have been using various webdings fonts but I want to make them transparent gifs. I woudl rather find a collection from the same author so the contour of the gifs blends together. So it won't look all jumbled together