In My Build A Business guide to Logo Design there is discussion of using icons. Of course if you aren’t an artist and a simple shape won’t cut it for your business type the next option might be using stock photography. This can be expensive or inexpensive depending on the resources you choose to use.
Inexpensive line art for logos can be found at:
More expensive, which can mean more unique designs:
I use istockphoto.com because it is inexpensive and after purchase line art it gives me vector art to use, which means I can easily change the colors and customize for my needs. You can download a free lo-res raster watermarked version to make sure the design fits before purchasing.
When using any stock images make sure you have read the fine print, the image needs to be legal for corporate use. If you are thinking about using a “free” image that you find online realize it is copyrighted and you are probably breaking the law.