There are several places/ways to get the icons you may be needing. Google search using the image tab will get you several options to start with. After finding a image(s) save it and open it with your graphic editor and begin the process of reducing it according to where you are placing it in the theme.
Here is a link to a site that has some icons.
http://www.wincustomize.com/Skins.aspx?LibID=39
another:
http://www.winmatrix.com/forums/index.php?showforum=48
another:
http://www.joejoe.biz/joejoe/load/
Hope this helps a bit?
Look on your own box you may be suprised at how many icons you already have? Convert them over from .ico to .png.
here is a small program that will let you convert .ico files to .png and .png files to .ico. Its drag and drop and easy to use.
http://www.mobile41.com/tools/Aveicon.zip
wrong thread 4 these but here are the matrix icons for the 71xx.
http://www.mobile41.com/tools/71xx_Matrix.zip