There are plenty of stock icons out there that one can buy, and for your purchase you will get a very large set of stock icons in many different sizes and formats. However, if you need to do icons on the cheap there are a good amount of free icon sets out there that you can use. Below are a couple of my favorite icon sets.

Thanks!
May I suggest the Tango Desktop Project as well, which base icons are very commonly used in open source software.
http://tango.freedesktop.org/Tango_Desktop_Project
I knew about the Tango project, I actually use them on my Ubuntu desktop.. I didn’t know if they were free or not. Definitely a good addition.
May i add http://www.iconiza.com