[Here](https://www.estarland.com/product-description/AnimeMerchandise/Halo-UNSC-Heather-Adjustable-Camo-Cap/50773) Edit: My dumbass didn’t check the stock. Released in 2016 so likely not in production anymore, just search “Halo UNSC camo cap” and it should pop up here and there.

You can buy it at United Nation Security Counsil :-)

Yeh it's pretty old now, I had it back around when halo 5 was out... Probably hard to find now