You can use open source art like Flare engine assets from here: https://opengameart.org/content/flare + https://opengameart....lare-collection + https://flarerpg.org/ + https://github.com/f...team/flare-game (just make sure to attribute the authors in the credits and check all the licenses to be at least CC.0 or MIT or some form of CC-BY)
Also Creative Commons 0 (public domain license) from anywhere can be used at least such as: https://opengameart....144&Collection= + https://itch.io/game...sets-cc0/tag-3d and just make sure you organize your art / game assets by license and also keep the artist names and make sure to keep them in categories if using many open source assets. Example of folder structure: Open Source Game Assets > License Type > Author and or Owner(s) > 'Open Source Game Assets Here'. Then attributes and sources and or links credits for everything to do your best due diligence to be with community good will and legally fortified.

For it just takes ~1 / 1,000 not legally amazing assets to legally mess you up and the entire work could be pulled and its ~90-100% legally sound even if its a transformative work unless you are ~110% know what you can and cannot do. So make sure if you get open source art make a clean organized art / game asset folder and sort by licenses (stick to CC.0 if unsure) and put artist names on folders of every asset so its organized and clean and legal.

Also put peoples names in the credits you cannot under-do this and it is very important for the CC-BY open source licenses that require attribution of the authors name of the open source assets being used freely. Also keep up community trust / good will and things should more or less super open up and there's like a million+ open source game assets you can use if you keep the legal open source stuff clean and organized. It's very great to have so much open source assets but be responsible and know your copyright laws and if you do all that you can have a super amazing library of forever free open source community driven game assets!
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Open Source Advice: Just stick to Flare assets and CC.0 assets and give the flare people credit and its like 9/10 awesome and ready to be super creative.