available online but most of the good ones come for a price. You may find resources to free ones as well but usually the quality is not all that impressive.Recently Abdylas Tynyshov of the User Interface Icons has floated for free download and usage, varied icons numbering a total of 560. All the icons have the PNG format and they are available in four different sizes such as 24/24 Pixels, 32/32 Pixels, 48/48 Pixels and 80/80 Pixels. As far as the license is concerned, they have been shared under the Creative Commons License. You can get the icons for download from User Interface Icons website.
The icons are all available in the PNG ('ping') format and have a nice transparent background. However the truly unique designs of the icons amount to only 75, the rest 65 are variations of the first 75. Variations include different connotations such as Add/Remove/Edit etc. They can be quite helpful for representing different status of an entity (printer, fax, clock, lock etc.)
Here is the link