Here are some websites, where you can find and download resources to add to the game.
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Pay attention to the usage license. Some licenses do not allow content to be used in modding. It is recommended that you use the CC0 (Public Domain) license if you want to publish your mod and make it available for all players.
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https://sketchfab.com/search?features=downloadable&type=models
https://www.fab.com/category/3d-model?is_free=1&ui_filter_price=1