Fomalhaut30: They aren't actually "dead". They're vat-grown and were never really alive. Just like most servitors.
AFnord: That depends on what fluff you base that one. In some fluff it is said that most servitors are convicts and/or other "undesirable" elements of society.
The Cherubim are pretty much custom-made.
Other Servitors come from three main classes. The first being vat-grown. The second is the convicts or others that were punished. The third being failed Space Marine initiates, though those tend to only be used within the Chapter that they were trying to get into.
It is, of course, based upon the world that the servitor comes from. Lotsa different worlds with lotsa different methods for creating them. Convicts are a good source since they'd otherwise be wasted.