Hey, I had the same issue (sold the purging wand to God knows who) so I decided to wing it...
I came in possession of a purging wand by doing the Dragon quest, but gave it to him.
"What a fool" I thought as soon as I realised my error so I continued searching.
I searched for hours for another purging wand (there is none), so I decided to go to the entrance of the Harbor
As I approached, the shadow of a Dragon flying above me, blocked the sunlight. While he was flying over me, he disabled all the Shriekers with a single spell... They fell to the ground, disabled.
Long story short: if you cock-up, do the dragon's quest and you'll be fine.
