Strictly speaking almost everything one could want to use a background is copyrighted in some way, by someone.
Question is; do they care enough to enforce it? Most probably not. Its not like you are a threat to their livelyhood or position as a company.
Try finding something useful at 4chan, for example. Be adviced that it is 4chan after all, take it for what its wort and read the descriptions carefullyor youll wander off into /b/ or /d/ land.
Anyway, if you cant find something you like, try making one yourself. Photoshop is magic, it does most of the work for you if you treat it well.
Button layout preview. Only useful for white backgrounds, obviously.
And heres the link to the transparent one. Just paste it on top of the image you want as bakground.
One last note; i snatched the image of the buttons from Jaxel and did some cleaning to get the transparent version.
Edit: If you dare to choose 4chan as startingpoint, try this section out first. Its the Anime/Wallpaper section.