Hey, welcome to Gamedevelopers.ie!
The best website I have found so far is:
You can be lucky or unlucky with the assets posted there, there are some fairly bad ones mixed in with some amazing ones. They’re all free to download, but keep in mind that "share alike" licences will mean that you’ll have to release your game as open source.
Have a look at user: Clint Bellanger’s stuff in particular; really good, especially his tilesets.
Hope this helps,