You can do a search in the forum to find similar questions. This for example:
https://forums.sketchup.com/search?q=where%20to%20get%20good%20textures
In addition to sites dedicated to providing textures, it’s worth knowing that if you see a model in 3D Warehouse, that has used a texture that you like, you can download the texture itself without having to download the model.
Can you give some examples of materials that you’re trying to find at the moment?