The safest and most reliable source for free assets is Reallusion itself. They frequently offer "Starter Packs" and seasonal giveaways to help new users get acclimated to the software.
: For free assets from external community sites, you typically download a .zip file, extract it, and then drag and drop the .iProp or .iMotion files directly into the iClone viewport. reallusion iclone 8 resource pack free