You might be aware of this website
http://www.spriters-resource.com/
It has a TON of sprites that you can use in your own personal/learning projects.
This can be useful for placeholder graphics or if you are trying to remake a clone of a previously loved game. However be warned, you can NOT use anything on there for commercial products. Do not even try it.
Read this part of the website and get it in your head:
http://www.spriters-resource.com/about/
I have quite a few emulators with 1000’s of games, I can see me using this to make clones in order to learn and practice game coding and design.
Hope you enjoy.