In fact, V-Ray for Rhino comes with some pre-defined textures, and there are free downloadable libraries online as well. This page will tell you how to find and make use of those resources on your computer. Two conditions are considered: Preinstalled materials and web-downloads.
Assuming you found an online library source, like this one, and downloaded a library of textures, what would you get? A compressed (probably "zipped") folder containing more folders with image, bump, and related files, organized into collections to represent various materials. After unzipping the collection, you use it in EXACTLY the same way as the preinstalled materials, but when the file chooser dialog pops up, you probably click "Desktop" or "Downloads" to find it on the disk.
Once you've imported a material from the downloaded library, the V-Ray Material Editor will return to it for subsequent material imports. That's why you need to remember "C:\ProgramData\ASGvis\Materials", so you can get back to the preinstalled library when you want to go back to picking materials from it.
When opening the Materials manager, most material texture icons will not load, or take a very long time to do so, and my computer's CPU goes into overdrive for some reason. Not sure if this is a missing paths situation, or if it's a missing library materials- though I do seem to have all the RVT 2021 material libraries and textures in my C drive in the proper location. 2b1af7f3a8