You can download Materials from the Indigo site. If you used the default installation settings then they should be stored here: C:\Users\
USER_NAME\AppData\Roaming\Indigo Renderer\materials\
If you haven't downloaded anything then the folder will be empty.
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.