"Cumriep sue" Selkirk, great to see another student of Khmer!
if you haven't done so, you may have some luck checking whether this is held by a local public or university library. I was able to locate a set of Vol 2 in a library in my country (Australia), and it is certainly of value in developing oral and written skills. BTW, I can't post it here, as it is an Audio-Forum version, as so not public domain. (And the audio is terrible in places, so I also hope for a good copy to be posted here one day.)
You probably already know this, but the audios for Huffman's excellent Modern Spoken Cambodian are available online, at
http://cls.yale.edu/cambodian/ , and the text is relatively cheap from Cornell, or even cheaper from a library.
cheers.