Newegg, TigerDirect, and many other online vendors as well as retail stores like CompUSA that carrry a large amount of pc accessories will see those. Even a local Radio Shack will carry something. Those are pretty common items and don't cost much.
Just look in the case itself at the bottom for the place and measure the diameter. Some cases will simply see tabs to hold the speaker inplace without screw holes near the front bottom.