The Crest is found in the room where you fought Yunalesca in Zanarkand. Just go up the stairs until you see the chest with it inside. ( Note : If you don't get it on your first trip here, you will have to fight Dark Bahamut first.)
Sigil : This is perhaps the most annoying of all the Sigils to get. Head to the Calm Lands and speak with the Chocobo-riding lady there. You'll have to do some training with a Chocobo, possibly; most of those are really easy, though. Eventually, you'll get the option to race here, and, during the race, get a negative time (under 0:0.0, and not including it). It IS possible. During the race, you simply must hit as many balloons as you can (-3 seconds each), but avoid the birds (+3 seconds each). Ultimately, it's just a lot of practice. Once you hit under 0 seconds, you get the Sun Sigil.