I use my own library, material available from other libraries, and various on-line sources - google books is very helpful.
I also use, as was suggested above, Google maps to try and get the geography correct.
My basic foundation is that if you're going to try and do something like this semi-seriously, you owe it to the readers and the "real" people who end up showing up in whatever you are writing to take it seriously.
Otherwise, just stick to the equivalent of Middle Earth or Far, Far Away and make it up as you go along.
Or make the point of departure so far back that the story being told does not involve individuals and organizations from actual history behaving a-historically.
Don't make the RCMP mass murderers who make the SS look like the Girl Scouts, for example - unless you can explain why, convincingly.
It is a lot easier and tends not to cause the problems with suspension of belief that things like, oh, a successful ZEELOWE do, from those who have some knowledge to base their reactions upon...
Best,