Mappers Guild projects have to use copyright-free backgrounds and I find UnSplash, Pexels, Pixabay and PxHere good options for this purpose.
You might also be able to find specialist hosting sites that provide backgrounds with limited usability so long as you ask permission from / credit the artist and it's not for financial gain, such as piapro (good for vocaloid-related works), or artists on DeviantArt / pixiv / etc. that specify their work is free to use.