iKnow.jp is the original source of these Core 2k/6k/etc going around for free on the internet. But yes, they’re a paid service. They used to be free and very popular when called Smart.fm. They have vocabulary and sentence pattern/structure SRS for Japanese, Chinese, and English languages. As well as custom decks made by users that use their really nice system, just like @nasatitan on this thread.
I made the following thread long ago about iKnow.jp:
It has information and screenshots of the service.
At least not officially made by the iKnow.jp team. I’m sure there are custom decks available uploaded by users, though.