Is it possible to still get Morostide items? And if so, how? Or will the opportunity come back around next year?

Do you mean from the extra quest (Cinthia and Quentin) prizes? They're still going until the end of the 29th. c: Morostide will be held again in October every year.

Oh, kind of. I mean like the trick or treat items, I've never participated in the Trick or treat event so I wasn't sure if I was still able to get items from the past couple of years.

Ah, I see! Well, the quest-a-thon is currently running (link here) so if you do Quentin's or Cinthia's quests you may earn some of those items. Otherwise you can buy the items you want from user shops or you can wait until October, when some old and new items will be available. c: