I discovered this method last year when I was in Asia, possibly when I was already in Cambodia. At that time I found multiple threads of various forums discussing it but I'll answer now off the top of my head and try to come back and add some links afterward.
- You can't do this on a tourist visa, but
- It's just as easy to get a business visa for Cambodia as a tourist visa. If you are eligible to get a tourist visa on arrival at the airport or border, you can also get a business visa there. The cost is only marginally higher. As I recall just $5 USD more.
- After entering Cambodia go to an immigration office. I can't recall if you must do this on the first day possible or if you can do it any time before your business visa expires.
- Get an extension on your business visa. As I recall you can get 6 months or 12 months and the fee is on the order of $100 USD or so. Some reports I have read that are now a few years old suggest that a 3-month extension is sufficient!
- Being in Cambodia on a business visa of at least a certain minimum length of time gives you some kind of legal status as a resident of Cambodia.
- The Russian embassy/consulate will accept this resident status as fulfilling their requirement for issuing visas only to residents of Cambodia.
References. Hopefully I can find some more/better/more recent ones to add...