Poliigon is described as 'Assets for achieving photorealism' and is an website. There are three alternatives to Poliigon, not only websites but also apps for. The best Poliigon alternative is Poly Haven, which is free. Other great sites and apps similar to Poliigon are ambientCG and Quixel Megascans.
Poly Haven is the most popular Web-based alternative to Poliigon.
Poly Haven is the most popular free alternative to Poliigon.
ambientCG (previously CC0Textures.com) is a free material resource for PBRs, HDRIs, models and textures licensed under Creative Commons Public Domain (CC0).
Quixe Megascans is a massive scan library. Megascans provide assets for games, vfx and much more. It's contents are 3D assets, vegetation, decals, tileable surfaces.
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High quality textures, but completely free.