'Royalty free' is a key term if you plan to use the game commercially.
'Public Domain' is a fancier term for the above.
"Royalty Free sprite sheets" came up with a lot of results.. beggers cannot be choosers so you won't find AGrade 3d models etc.
You could even BUY resources! :)
Deviant art- a lot of spriters (not sure what the right name is) on there who sell their work for commercial use..
But then in theory, if you never plan to show anyone... and it's product which you are testing things on... you *could* just rip some art and use it. BUT you can't claim it's yours or legally use it commercially :-)