A good first place to look is the Image collection by VZTech. My next stop is the large Icon image collection I have collected over the last 5 years. If you do not have such a collection I would recommend a trip to Everything Icon which has the largest collection of icons I have ever seen.

If you find an Icon that looks good but you need it for a toolbar image then you can always resize it using an image editor. This works for images that are not extremely complicated, but most icons should convert to the 16/15 size without loosing too much image quality. The trick is not to use the built in resize function of the image editor as they always tend to focus on the darker colours and tend to loose much detail.

The way I convert the image is to load up the icon and convert it to a bitmap. Then do a save as under a new name. The first thing to go is any white space around the image. This normally saves a few pixels. Then taking each axis in turn look at the image and say "which line of pixels will cause the least amount of loss?" Then copy and paste the part of the image to the right or bottom over the line that can go.

Its always easier to start with something that looks like what you want and alter that, try combining pieces from two or three icons to create a new icon. If after all that you still cannot get it right then ask your self do I really need an Icon here? As it is bad GUI design to include an image that has nothing to do with the word or phrase you are trying to convey.

If the answer is yes then follow this golden rule: Icons should always mean the same thing from application to application and window to window, NEVER use an image to mean different things in different applications. Therefore pick something that does not mean anything like a fractal, you should save the other images for other applications. By picking something off the wall you are helping to make sure that you do not confuse the user at a later date in a different application when you could use the good icon to mean the correct thing!