You can make them with OpenOffice.org Writer.
I did it once just to see how it worked.
OK, I am using the ODF 1.2 Specification Part 3 because it is not too large but has several chapters.
So I opened OpenDocument-v1.2-cs01-part3.odt (actually using LibreOffice 3.3.2 but OpenOffice.org 3.3.0 and earlier work the same way).
On the menu bar select File | Send > Create Master Document
This will give you a Name and Path of Master Document dialog, with Save as type set to ODF Master Document (*.odm).
I navigated the Save dialog to the folder where I wanted to save the Master and its parts. I then specified MasterDocDemo. There's a Template setting pull-down which defaults to Outline Level 1. I left that alone and clicked "Save." I ended up with an 8-part document:
MasterDocDemo.odm
MasterDocDemo1.odt
MasterDocDemo2.odt
...
MasterDocDemo7.odt
Do that yourself with something that has chapters or something with headings at Outline Level 1 and then explore.