I think what you’re asking for (a big repository of pirated STLs) doesn’t exist because it isn’t really necessary or desirable.
There are a LOT of free models out there. Thingiverse, Printables, Thangs, etc. You really don’t need to buy models if you don’t want to.
Most of the owners of paid models are just random nerds, not the huge mega corporations that own movies and TV shows. And the printing community is very small and supportive, so the general sentiment is that if a model isn’t free, it’s probably worth paying for.