So in case you didn't know, there is a Sack of Spectral Spiders Pickup Trick. This Pickup Trick is going to allow you to bypass the limitation which ensures that you will be guaranteed to get the Sack of Spectral Spiders, during this event.
Before I teach you the Sack of Spectral Spiders Pickup Trick, I want to give you the guide on how to get the Sack of Spectral Spiders, period.
Sack of Spectral Spiders Pickup Trick
First of all, complete all 4 Halloween quests in your garrison, and they will reward you with 5 seasonal tokens. The NPC is called Izzy Hollyfizzle and the quests are done in Shadowmoon Valley on the Southern most island. It's a good idea to farm some mobs in that area, as there is another toy which drops from these mobs "Coin of Many Faces".
When you complete these quests, you can buy the item "Creepy Crawlers" from Izzy Hollyfizzle. That item will spawn 3 different battle pets (all with one breed, for pet collectors) and 1 rare mob called Arachnuis near fishing area. If you kill Arachnuis, this mob has a chance to drop Sack of Spectral Spiders. But the problem is you can only loot this mob, once a day.
This is where the Sack of Spectral Spiders Pickup Trick comes in...