Write to RCMP HQ in Ottawa and to the depot in Regina, SASK.
I got an 8X10 color photo of a constable guarding a rail shipment or something years ago and a photo of a M-10 next to a ruler. They added a pic of a shrouded Colt .38, Cobra or DS. Also used Bodyguards, M-49, prob. other S&W snubs.
Interestingly, the Inspector in Ottawa who replied to me had my last name. I don't think we're related. Prob. just a cooinkidink, as my daughter used to say.
I'm pretty sure I know who you are, and I told you on Gunboards to scrounge photos from Public Information Officers in the military and police of the countries that interest you. Military attaches are also possible sources. Write to them at their embassies in Wash., DC. Some will connect you to suitable museums.
Ask about copyright, but these govt. supplied PR photos are usually not going to be a problem, although you may be asked to credit the sources. Some photos on Wiki are also usable with accreditation.
Avoid the American Museum of Natural History. I wanted to get pics there for an article on Roy Chapman Andrews and his guns. They wanted so much for repro rights that one photo would have taken everything that I hoped to get from, Gun Digest. I scrapped that article idea. BTW, I have no idea where Dr. Andrews's guns are now. If ever sold at a major auction, they might well bring as much or more than Jim Corbett's .400 Jeffery rifle did a year or so ago.
Not that the Museum would have pics of Mountie guns, but you might want to write an article about Andrews or Wendell Phillips, etc.