When creating your own maps, it's always better to not use copyright-protected assets! Yes, that grass texture from a quick Google search may look kinda cool, but chances are that you're simply not allowed to use it.
This (non-final) list contains 100% free-to-use resources licensed under the (very permissive) CC0 license.
What is a CC0 license?
CC0 is short for "Creative Commons Zero" and means that the original creator "has dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law". In simpler words: "You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission".
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https://creativecommons.org/publicdomain/zero/1.0/deed.en
Textures
HDRI Skies
Sounds
3D Models
Misc