Using google came up with this one at PCLoft.http://www.pcloft.com/casespeaker.html
This is just a Speaker from Radio Shack. $2.79 for an 8-0hm mini speaker 29 mm ( Approx 1.1 in). You would have to add a connector (or suitable female pins to it to connect to the mother board).http://www.radioshack.com/product/index.jsp?productId=2062406&cp=2032058.2032230&cp=2032058&f=Taxonomy%2FRSK%2F2032230&categoryId=2032230&kwCatId=2032058&kw=speaker&parentPage=search
I have a feeling that Newegg might have, but, if so it is buried as I could not locate and Google only came up with one source. They SHOULD NOT be so hard to locate.
Do not know if a computer speaker is 4 ohm or 8 ohm. If unknown, pick higher ohm value