Android 6.0 Marshmallow is expected to hit device in the third quarter. You can still download Developer preview of the latest version of Android OS. It is suitable for developer only, else you may end up corrupting your device software.
However, you can still get the Android 6 Marshmallow official stock wallpapers. We have got all the wallpapers for you, and has posted the samples here. If you like them, download all the wallpapers in zip file clicking the link given in the end of the article.
These samples are in very low resolution, so download using the link only for the best quality