It's free real estateG 15

You can discover bivouacs by stumbling across them or looking for the smoke-like signals indicating where they are, but I think the easiest way is by looking for the intel icon (the little “i”s) and selecting a nearby bivouac whenever that’s an option. (Instead of a weapon blueprint, collectible, etc.)

No matter what this will take some time to get, but easily accomplished during a playthrough.