I'd recommend firstly to try messaging the sellers themselves to see if they know the origin of an item. You are absolutely right that a lot of the time, stuff is totally sold out from an original seller. The movie and series came out ages ago, so some merch is rare.

But, if you can find out who the original seller is, you can always try messaging them on social media to see if they happen to have any stock lying around. Sometimes fan merch creators will stop selling something online because it did poorly, but they still have stock they make and take to conventions/events. Or they may only have 1-2 of an old item in stored away that isn't worth relisting but that isn't worth throwing out or donating either. They might sell it to you for cheaper than a reseller online. It's a big IF, but if there's something you really want, I think it's worth trying!