Hi Kelly,
That's a great question. You can't actually find the buttons sets you created. The purpose of creating a button set on a slide is so that when you have more than one button on any given slide, if you make them into a set then they behave like a set. For example, if you select one button, it changes to a selected state, and if then you select a different button, the first one changes back to normal and the second one you selected changes state, that sort of thing. But you can't really access that button set from a menu.
What you can do is just copy and paste a button set from one slide to another--just watch for any triggers you might have added to the buttons to see if you still need them on the slide onto which you are pasting. You could also have a hidden slide where you can keep these repeatable objects and just copy and paste from there.
Hope this helps!
PS: What you are asking about would make a great feature! Feel free to submit it as a feature request here:
https://www.articulate.com/support/contact/submit.php?form=feature