In a beeper store!
Seriously, you should be able to find numerous online vendors that sell those little piezo pc case speakers. All I did was google it and the first link to come up was exactly what you are looking for:http://www.cwc-group.com/casp.html
Quite a few of the newer pc cases come without speakers. However, the last motherboard I purchased came with one. Other motherboards use red and green led's instead of a speaker.