And what if there's no next copy?

Hit up a forum, a discord, or an IRC group and request someone post it. Or just put it in a queue and wait.

people sometimes make it seem like you'll be able to find everything you want and nothing will be dead.

I mean, generally. I have a roughly 250TB datahouse, with about 800GB of outstanding requests I can't find yet. That's 0.32% of total requests. This is with strict profile matching search criteria though, so I'd probably find even 50% of the stuff I wanted if I let up on the requirements.

Not many indexers even have a request system so it's not like you can even make a request and hope someone uploads it.

This is what the 'Arrs are for. They work great, and are super reliable. For full automation, you can input a source list from mdblist with criteria you specify.