The Center for Government and International Studies at Harvard (CGIS) librarians dug up some digital map collections for me. Here is what they sent:
One person you want to talk to is Damien Evans at Univ Sydney, who directs the Greater Angkor Project. Despite the historical emphasis they have also done some interesting Remote Sensing work on Cambodia.
GADM provides basic vector data for Cambodia (and all other countries). You could start by getting the countries of interest in SE Asia.
You may try to contact CSEAS Library, Kyoto University, or the developer, Xianfeng Song.
Geoinformatics Inst of AIT has data, see
mekong river commission info page for Cambodia documents
dated study with good overview of resources
I also have a bibliography on development of the Upper Mekong (in China), which is quite old: