I plan to get the Secret of Metamorphosis and the extra Color Set for Hair (is there one for eyes as well? I like to have a character with red eyes) but I believe they cost a lot, I don't know how much so that is why I like to know how to earn a lot of Rift Crystals as quickly as possible.
"Invoke... VIRUS SOUL!!!" ~ From my Virakins global manga project Official Husband of Caster of the Fate/Extra boards
Goblins usually drop some rift crystals upon death and larger mobs sometimes drop rift clusters. Easiest and Hardest way is to just try and get your pawn rented, best way to get said pawn rented is to make sure they are well geared, unique, and useful for the class they are currently in.