I want to be able to crouch anytime, not just when I get into the bush, so is there a button for it or something?
From what I can recall, you can't. I haven't been able to find out if this has changed since I played.
Here is a quote from Steven Masters (Lead Game Designer) I found on this Kotaku article:
our stealth is primarily "social stealth", and we've been debating having a crouch button since pretty much day 1. It was always the vision that crouching in public spaces is not "hiding in plain sight"—if anything you're calling attention to yourself. We found that the "stalking zones"—the low vegitation where you can crouch down while low profile—are a good compromise since it allows you to crouch but only in circumstances that make sense.
Looking a little closer, it was in response to this question on the Reddit AMA.
Doesn't look like there's a way to crouch outside of vegetation.
In short, you can't. see list of controls from gamefaq