Most of the more common ones can be found in this thread: RPG Maker, Resource Thread
Just about everything there is free to use, but double-check for any sort of licensing if the poster didn’t explicitly say as much. If there’s sprites you see commonly in other games but not that thread, ask those game creators where they got them, if they’re free to use or edit, etc. For the most part, if you’re allowed to use them you’re allowed to edit them, so long as you don’t try to claim they’re completely your own work.