Throughout Night City you’ll find loose pieces of legendary armor, most of which is just lying around for the taking. While a few pieces of legendary armor can be found in Watson, most of it will lie in other district in Night City - you’ll need to complete the job The Heist to gain access to other districts outside of Watson. Some might also require you to do some platforming to reach (the Reinforced Tendons cyberware will make this much easier) while others require certain attribute checks to access. Most, however, can be found right out in the open - their only defense against you being Night City’s sheer size.
While their locations are static, keep in mind that almost nothing else about legendary armor is. Like most equipment, its level will tend to scale to your own, so picking up a piece of armor at low-level will yield a low-level piece of armor. More importantly, the number of mod slots each piece of legendary armor has is randomized when you pick it up. Inner and outer torso armor will spawn with 0-4 mod slots, while every other piece of armor can have 0-3. Given how powerful mods can be, spawning a piece of legendary armor with 0-1 mod slots is tantamount to wasting it. Fortunately, you can just save near the armor, pick it up, see if it has the correct number of mod slots, and if not reload and pick it up again until it does.