Where do companies get their button graphics from? [on hold]

I'm just wondering, where do companies get the graphical elements of their software from? I know some projects create their iconography by using websites such as "noun project" (e.g. TAILS Linux).

Are there any other places where you can get all of the elements for your UI gratis? To be more specific, I'm looking for quite low-res button graphics as seen here:

They all seem to be very similar, are there any conventions laid out by Microsoft that I could follow if I have to design them myself?