Here are free assets that you can use in your addons. Source 2 uses PBR, so find PBR textures. If you’re using Source 1, you need regular pictures, not PBR.

Textures:

FREE:

https://cc0textures.com/

https://texturehaven.com/textures/

https://sites.google.com/view/mtextures/m-t-e-x-t-u-r-e-s/photogrammetry-textures

https://www.textures.com/

PAID:

https://quixel.com/megascans/home/

Models:

FREE:

https://store.chocofur.com/search/free

https://www.blenderboom.com/product-category/free

https://archwizz.jimdofree.com/

https://polyhaven.com/

https://quaternius.com/

https://mantissa.xyz/pages/free.html

https://publicdomainmodels.com/

https://www.3dmodelscc0.com/