I would like to add a "share" button in my Mac app, like Safari and other apps have in OS X 10.8.
Is it possible to get this icon programmatically from another bundle on runtime? Or should I make a copy of it and include it in my app bundle?
[NSImage imageNamed:NSImageNameShareTemplate]
will give you an NSImage with the default share image. You don't need to include a copy of the image since it is a system resource (at least, in 10.8. It is a different story if it is 10.7). There is a list of other system provided icons here.
You may also want to look up NSSharingServicePicker
if you will be implementing a standard share menu.