Stock Id's and Icons on buttons

[Perdie Perduta <rsperduta gmail com>]

I'm using two GtkMenuToolButton and one is to select an audio input
device and the other to select an audio output device.
I can't find a suitable stock id and so at the moment just put in
gtk-missing-image

Can any one help me understand how to add my own stock id's or icons
to these menu buttons, preferably in Glade rather than having to code
it in the software.
also if there are add on libraries of extra stock items on the web.

Thanks in advance :o)