Hacking into a Lab is quite difficult - but finding a lab key on zombie scientist or a wiped out group of scientists in the wilds is usually pretty easy if you keep an eye out for them, and that’ll get you right in.
You can also blast your way in, but I really can’t recommend that.
You can also drive around the world a lot and just look for hospitals - they’re big, but not as common as they used to be, it seems.