You're gonna have a hard time once you're fighting in skyshrine looking for one specific item. In general - loot tables in skyshrine both for cubes, wurms, guardians, sentrys and fe'dhars are huge with alot of items but most of the items being very to ultra rare.
That said - the tiny coin purse is one of the more common drops. The other, larger coin purses are much much rarer and I think only drops from nameds/high levels.
Your best bet is to pick a tiny coin purse up in EC for 300pp or ask one of the numerous necros camping fe'dhars to give you a tell if they have one rotting (which they will have).
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