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You can find the images here
website-root-folder\modules\_protected\Shell\EPiServer.Shell.UI.zip\8.5.0.0\ClientResources\epi\themes\sleek\epi\images\icons\
I am not sure whether you can get these images programatically, if that is what your question is.
Is it possible to retrieve the icon used in the page tree for a specific page? For instance, in the following example, if I have "Start" I would like to get the home icon and if on "How to buy" I would like the folder icon: