You can turn in yellow scrips in Idyllshire, Rhalgar's Reach, Tamamizu, Gridania, Ul'dah, and Limsa Lominsa.
So if you haven't found anywhere to turn them in you can't have been looking hard.

The level 60 red scrip gear is good enough for being able to earn yellow scrips. If you haven't unlocked Custom Delivieries yet you really should do so - it is one of the better sources for both Red and Yellow scrips.

There are plenty of player created guides for end game gathering and crafting. Google is your friend.