Suppose there's China/alibaba but not sure how easy it would be to arrange to get them shipped. I was part of a group buy of items coming from overseas and the person here had problems with ship time and broken items. Had to pick them up at a port too I think. Probably tariffs to be paid and documentation.
Anyway, Have your tried calling Dollar Tree directly to inquire? Not the local store but the department who does the bulk ordering? Getting them through them directly might be the simpliest. At the very least they might have an idea of when they might be getting in their orders for halloween.
Not sure if this might help be sometimes the individual stores keep leftovers in the back. I got to know a new manager at one of my local stores who had a couple of leftover cases of bagged creatures in back she'd sell me out of season. She was happy to make space for other things.
I will say be careful what you might order off season. A few years back I needed a bunch of DT Rats in July. They still had them listed online at the time BUT to get the 2 cases I needed I would have to pay for them to be shipped doubling their cost. This was a few years ago and maybe their policy of things stored in their regional warehouses can only be shipped from there at your cost instead of sent to the store for free pick up has changed.
I will add if you happen to know the store manager of you local store and they HAPPEN to have a box or 5 sitting in the back collecting dust and taking up space you might be able to get them to sell to you out of season. I'm told they aren't suppose to but my local manager at the time needed the space for current stock and was willing to sell me boxes halloween creepy creature in July.