Where can I find the source files of eclipse icons?

Eclipse has a User Interface Guidelines wiki page, where it is described how to create your own icons. Also, it includes several templates of the common elements, which you can use for base of your own icons:

• Common elements
• Core wizards elements
• Core icons elements
• etc.

There is a chance that the icon you want is not available, but it is worth a try.

There is a GIT repository at Eclipse with SVG sources for many of the icons: https://git.eclipse.org/c/platform/eclipse.platform.images.git/

It doesn't have all of them, since the SVG version of the icons started as an individual project, and then it was adopted by Eclipse. However it's a great source to create your own icons that follows the Eclipse style.