I bought some of these cap screws fron small parts and bearings here in brisbane they are about $2.20 each. I also tried a stainless steel one from the same place for about 80 cents ea which also work. try the website that was posted by ghaden.

Also maske sure when you are assembling the diff that the thrust bearing is palced in with the bronze ring u see in the centre is not under the head of the screw as this binds under the head and tightens the screw and snaps it.