The drop rate is quite low (think loooooow
). I think I only had one or 2 spawns, so gave up on this method and chopped the reef.. I wouldn't keep buying the Cursed Reef as it's very expensive. It does drop from the Tidegate pretty often, so if you can stand waiting, then do. If you're like me and want to finish it as quickly as possible, then pace yourself with purchasing them. Once you've finished the storyline it drops nearly every collection from the Tidegate, so you can soon amass lots and lots of them in storage.
Adira also drops them, so collect from her once you get her. Won't help you quite yet though...