The Sun Crest can be found in Zanarkand, right after defeating Yunalesca in the main story. Once you’ve beaten her, there will be stairs going down in the back of the room. Take those stairs and you will end up back in the same room. Head back to the staircase in the back, but don’t use them this time, as instead, look for the chest near them to find the Sun Crest.
It’s important to remember to grab this on your first run through the place, during the main story segment. If you don’t loot the Sun Crest then, upon returning to this room later on, after obtaining the use of the Fahrenheit, you will find Dark Bahamut in the same room. You will have to defeat him in order to get past him, which is some postgame/endgame stuff, including a lot of grinding to improve your stats to the near max values.