Everyone makes the crest and sigils easy to get, well i guess about 3/4 of them are but the other 1/4 are hard im only going to put down rikkus and yunas since they are the only weapons you have now
(SPOILER)
rikku's- Mercury Crest: in a whirlpool located by the adviertisement in Sanubia Desert
-Mercury sigilo to south part of Sanubia desert into the cactuar stone which will tell you to find ten cactuar. It will give you a hint where the cactuar are then you go and find the cactuar in the desert and sneak up on it
Yuna's- Moon crest:located on Besaid beach
- Moon sigil: defeat all of Belgmine's summons at Remien Temple
hope that helps