Sure, there's tons of materials out there. Some here in the free stuff, some on webring pages, etc. Download the .mat that you like, put it in a download directory, unzip if needed. Then go to the materials editor and select the library where you want to keep the material. Click the word import and select the .mat you just downloaded. Bryce will add the .mat directly to the database in directory you chose. You won't need the downloaded file again unless you crash and have to reinstall from scratch.