Where can I buy assault rifles?

You can purchase them at the Black Market shop in Darwin. The shop requires a password to enter, or you can kill the guards if you get the password wrong.

I don't know if this is the case with this password, but one thing to be aware of about the passwords in this game: If you go through the paragraphs, you might find one with a fake password. Using the fake password might let you in, but will likely have some side effect, and it might not be immediately obvious. (It won't prevent you from completing the game if your party isn't already in bad shape, I believe.)

How do I get the password?

I think the bartender in Darwin will tell you if you ask him about the password.