In each different Uncharted game, we see Nate Drake wearing a different belt buckle. Each buckle corresponds to a different theme pertaining to that entry in the Uncharted series. Below is a list of the buckles identified thus far in the games.
A skull-and-swords belt buckle reminiscent of a pirate's jolly-roger. Silver and black in an oval design. This exact belt buckle can be purcahsed at itbuckles.com.
A rising-sun design in a clamshell-shaped buckle, appropriate for the Tibetan setting of the game. This buckle seems to be a more golden color than the previous game's buckle. This exact buckle is not available for purchase, and the photo below is from a replica prop maker.
A horseshoe belt buckle with a western pattern on the outer edges. This buckle is a golden color, with a rectangular design. With this game, being as it is Nate's most dangerous and personal adventure to date, the lucky horseshoe seems extremely fitting for this entry. This exact buckle came with the Uncharted 3 Collector's Edition, and is available for purchase on Amazon.
The belt buckle in Golden Abyss is a dark silver belt buckle, with a square/rectangular design. The design of the belt buckle is similar to Nate's buckle in Uncharted 3 except it is of a sharper rectangular design. The inside of the belt buckle is comprised of multiple patterns surrounding a circle that is emblazoned with a very large scorpion. This fits with the setting of the game which is in South America. This particular buckle is not available for purchase however many similar belt buckles can be found on Ebay and Amazon.