Theres a few ways to skin this cat depending on device you wish to use. I tend to tour with a garmin and a android phone and am a bit of a mapping geek so will chime in with the little bits of info I know.
Starting with a simple android cellphone. Pluses are, many people have one, so no hardware cost, minuses are screen isnt always great, it chews through the battery, and its not weather / shock proof.. However as a casual glance to check your on the right path, its great. Also, as outside of town Cambodia has a fairly limited road network you need to simply get on the right path more than in more complex layouts. Hence a Phone setup is less of a drawback than in some places.
Of course most have google maps as an out of the box app, but thats only going to work while the data connection works and is fast (shockingly in lots of very rural cambo 3G does often work.. I can have decent quality 3g skype video calls with Kampong Cheam province to a village that doesnt have electric !!). Theres also a slew of apps that pull map data from many online sources. The OSM collaborate maps being one of the more interesting options. The few I have tested and that are worth knowing about are..
An app called Maps(+) for the paid version and Maps(-) for the ad supported one. Both apps allow you to use google maps, or possibly even better for Cambodia the OSM map tiles. Also you can 'cache' map tiles to your phone over a wifi connection so as not to drain 3G data or not have map access while the data connection drops out.
I used to always cache up a city I was going to at med to high res and the country at low res and that way I have the motorways and main rds of the country and the basic detail layout of a city all in my hand even if I didnt have a 3G connection.. Handy bit of pre prep and great if heading for a weekend to a new country that you dont have 3G in or dont want to pay 3G data roaming charges.
Last one I have tested in the cellphone side is oruxmaps a really rather slick app that I have only recently found. It can use 100's of map sources, looks great and appears to cache offline too. The really nice thing about oruxmaps I have just been informed of is the latest version will load a garmin .img file if the map is fully unlocked (which can be done to any map

) and provide garmin style mapping. Like I say this is a new app to me but I will be testing it this trip.
Outside of the cellphone use and into dedicated GPS units Garmin seems to be the default choice for global mapping. I have a bunch of different maps for Cambo / Laos / Thailand that I can make available but the best Cambodian one I would rather not post online. I am not even sure of the OSM map export has overtaken the routable garmin map I have in terms of detail, its progressing so fast on OSM I havent had a chance to test it in the last few months. I would love some feedback.
I currently have for Cambodia
LaosCambodiaVietnam-R6 (think a Mapsource)
Rottweiler-v2 (Mapsource)
Cambodia v3 (IMG garmin format to load a GPS directly) << The best one IMO
I dont want to put the last one hosted online but can find a way to get it to you directly. Or can arrange a place you can pre load it from in Phnom Penh. They are not huge, under 20 MB for the v3 mapset.
I would be really interested in feedback on if the v3 one above is still superior in roads and routes to the OSM project.
I also have a whole bunch of Vietnam / Laos / Thailand maps in mapsource or IMG formats for those continuing on one way or another. Just shout and I will see what I have.
Lastly and its so obvious it goes without saying.. NEVER just trust a GPS with these kind of home made maps, always look at the routing in full before setting off and use common sense on which way its sending you, sometimes a tiny glitch in a junction can make the route guidance think 2 rds dont link and send you on a huge detour when in fact they do and its simply machine logic not working. Dont ask me how I gained that knowledge

Hope some of that is helpful and hope to meet you guys before you depart.