The days of grabbing a random ISO from an open directory are over. The risk of ransomware, credential theft, and botnet recruitment is simply too high. While the "index of" search trick is a fascinating piece of internet archaeology, using it for operating system files is like performing surgery with a rusty knife.
Leave the "index of" directories to the data hoarders and security researchers. Your identity and bank account are worth more than a five-minute download. Index Of Windows 7 Iso
A typical result looks like this: