Some guards and civilians can be pickpocketed for various small amounts of coin, ranging from 5-30. However, during Mission 5: Lady Boyle’s Last Party, you will have the opportunity to pickpocket nobles that have very large sums of coin on them. Due to this mission also involving the Well Mannered trophy, it is best to make a manual save prior to attempting to pickpocket as well as learn Bend Time (level 2).
Once you have saved and have Bend Time (level 2), locate any of the Lady Boyle sisters that are walking around the mansion. They are wearing very distinct black, red, and white outfits. Each one gives 100 coin which will let you complete the trophy in a matter of seconds, rather than painstakingly pickpocketing dozens of enemies.