I've seen it in game at least once. If I remember correctly.. it was available in caves at the start of the session, then cave loot was nerfed and it's no longer available there.
I'm not sure if it's been made available in lockers, I certainly haven't found it there.
Note: I'm talking about Enclave Armour (the modified combat armour), and not Advanced Power Armour (the armour that Enclave members wear).