Indiana Jones and the Great Circle's disguises just help the game go down smoother, freeing you from dealing with its tedious stealth and letting you focus on exploration and puzzle solving. Prioritizing getting the best disguises as quickly as possible is 100% the move to best enjoy this game.
In the game's first zone, the Vatican, the outfit that will let you access restricted areas is the Blackshirt disguise, which can be unlocked only after progressing the main quest far enough. It is also easier to find on your map if you begin the Underground Boxing Ring sidequest, which will give you a quest marker for the disguise.
Where to find the Blackshirt disguise
The outfit is located in the far southeast of the Vatican map, in a courtyard adjacent to the Apostalic Palace. Until you get far enough into the Vatican's main mission, this courtyard will be blocked off with impassible doors.
Progress The Stolen Cat Mummy to the point where Cardinal Antonio gives you a bottle of wine and directs you towards the Tower of Nicholas V with the objective "Find the Underworld Gate" (don't confuse this with a similar objective from the Mad Priest side quest, which sends you to the sewers underneath the tower). What follows is a lengthy underground tomb raiding level under the Vatican.
When you emerge topside at the Apostalic Palace excavation site, you'll have a new objective to follow a nun who opened one of those locked doors I mentioned. Here you'll be introduced to The Great Circle's female lead, Gina, and watch a cutscene of a Zeppelin docking at the dome of St. Peter's Basilica.
After this cutscene, you'll be on a balcony overlooking a courtyard where some Blackshirts are doing laundry. Take the zipline to the opposite balcony, climb down, fight the Blackshirts, and the disguise will be yours. While you're down here, one of the extra inscriptions you can photograph for Antonio is at the opposite end of the courtyard from the uniform.
How to start the Underground Boxing Ring
The boxing ring is located to the west of the main drag of the map and north of the Borgia Courtyard with the entrance to the Sistine Chapel. There's a doorway behind the stairs up to the Fountain of Confession, and the boxing club is down the stairs inside. If you come here before finding the uniform, you'll get a marker on your map showing the location of the Blackshirt disguise. Once you're suitably garbed, the boxing matches are a great way to earn cash, you'll find a dispenser of infinite healing bandages, and there's a vendor selling combat-focused skill books as well.
