Google images, Deviant art, various sites depending on different themes that you'd like to see. Some of which we probably shouldn't specifically mention in public
You can set your search settings up so that you'll actually find more potential images.
You can right click and view original image or save image, rather than clicking a download button on a site.
If the site doesn't let you see the original image, you probably don't want to use that site anyways.
Many wallpaper sites like to put their address somewhere on the image to advertise themselves, or remind you where you got the image. I don't like those, so I usually avoid them.
Be prepared to filter through a ton of low quality images that were scaled up from lower resolutions, and images with odd proportions.
It's better to find a larger image and scale/crop it down to your needs. However, there are also programs that can upscale your image with nearly lossless results.