Lady Boyle's last Party (Misson 05) is a good point to start/finish this Achievement. Just go into stealth mode (press B) and then you can Pickpocket anyone (press X). They don't give a fuck if they see you stealing so its easier then Pickpocket Guards, loot 2-4 pockets and the Achievement should pop.
You may watch the Full Mission 05 Playthough (Pickpocketing at 7min, Youtube Timelink:
http://bit.ly/Pmp8ig )