A lot of the time they're niche specific, I would have found out about a lot of smaller sites from reddit and then just switched to searching them directly once I had a better understanding.
e.g. every second post in retro games subs was always "how much is this worth" and the answer is use pricecharting.com, analog camera subs or analog repair invariably reference butkus, there's heaps of small archives, wikis and old community forums out there for anything you can imagine.
If there's specific niches you're interested in maybe ask in those forums here, they might be able to point you somewhere.
Haunting_Tale_5150
rosatherad