Buy safety equipment from your home country and always wear it, no exceptions I saw loads of fatalities over the 8 months I was there last trip.
My first place every single time would be..... Lucky Lucky in Phnom Phen Cambodia you can get a bike without being a resident of that country, with all the documentation you will need for leaving the country. He has loads and loads of stolen bikes from Japan, I even remember seeing 4 or 5 1150GS's around the workshop and his staff know their sh*t when maintaining them.
In Thailand unless you are buying from another Tourist you will have to be resident to get the paperwork (can take months and costs loads), but also beware of the bikes import date as you might have to pay overstay on it.
Japan way way too expensive.
Laos, Burma, China you will not be allowed to buy a bike and take it into Thailand.
Malaysia, Korea I don't know as never asked.