During Chapter 9 of The Last of Us Part 2, you will have a chance to wander around downtown Seattle, and you can find an Antique Ring in this area. But getting to it can take a bit of time, and you will need to learn a vault combination to unlock a locked area to receive the item. Luckily, all of the information to receive the ring are in the same area, inside of the Westlake Bank.
On the southern portion of the map, you can find Westlake Bank during your first day in Seattle, pictured above. Leave your horse when approaching the bank and proceed inside on foot, going through a small cavern area. Follow the path until you can jump down a small ledge to enter the bank.
Feel free to explore the bank, but the Antique Ring is in a room to the back on the left side of the building. When you first enter this room, there will be an item Ellie can interact with on the ground. After opening it, you will receive the code for the large bank vault in that same room, 60-23-06. Approach the vault to input the code, and Ellie will unlock it.
There are large piles of cash all over the bank vault, and you can find a pump shotgun on one of the dead bodies. In the back of the vault room, on the right side in the corner, you will find a locked box you can interact with. The Antique Ring is inside it. You will receive the So Great & Small trophy once you have it.