Library is the big instance in diamond shores that has lvl 51-55 mobs and quests. Mobs there drop pieces for T3+ Obsidian weapons.

Mobs in Exeloch, and Sungold Fields drop mats for T1-T3 Obsidian weapons.

Quests in old areas that were for levelling to 50 wont be much help for 51-55 unfortunately.

For who could be free when every other man's humour might domineer over him? - John Locke (2nd Treatise, sect 57)