Might not be the best way, but you can find most of the icons used in various dijits (like measure icons, draw, basemap thumbnails) if you go to http://jso.arcgis.com and select a bunch of modules (or all of them, maybe!) and do a custom build. You will find a slew of png files in the zip file.
If you are using a Dojo theme (e.g., Claro, Soria, Tundra, Nihilo) you can access these icons using classes. For example, the disk icon (save) is accessed by adding the following classes to an HTML element: