@anye go to any avatar world and look up avatar you want, before you select the avatar open your AppData folder by Win key + R and type appdata, and go to this file AppData\LocalLow\VRChat\VRChat\Cache-WindowsPlayer, once you have them opened, check your time and then select the avatar, when they are done downloading it will appear in the cache folder, make sure to select the newest file that appeared, go to folder of the file and get the __data folder, that is the vrca file
as for private avatar that is another story