There are a few ways actually. I have a selection of sites which are full of crazy and sometimes I can find new topics by looking at the links people post on those sites. If I'm using Google, there are at least two good ways to find material.
1) Start generic and narrow it down. What this basically means that if you want to research, say, Harry Potter, you would start with something really simple, like "Harry Potter forum". It usually only takes a quick look to notice what's the lingo of the fandom/community, which you can then take advantage to narrow down your search. For example, if we take the ship Harry/Hermione, aka Harmony, we could make a search like "Harry Potter+Harmony+stupid" or "Harmony+shipping+bitch". This would then give you a much better (?) sample of the type of fans you're looking for. This method can also be combined with method 2, which is
2) Understand that people are not creative. There is a tendency for people to repeat themselves, quote one another and use cliches in regular conversation. Thus, you only have to think of what type of person you are interested in and then try to figure out how that person would speak about a certain topic. For example, you are researching conspiracy theorists. Now, there are many types of people who belong to this category, so you're going to want to narrow it down. Say you're looking for nutters with an elevated sense of self-importance. You could make searches like "UFO+government+sheeple" or "aliens+'trying to silence me'". If, on the other hand, you want to find douchebags, you could go for something like "paranormal+"my two cents" or "TRAPS+'I'm a ghost hunter'"
Sometimes finding something is easy. For example, finding JustRage only took a single googling for rant+fucking parents. On the other hand, to find some real niche stuff you're going to have to dig through the generic shit first.
Well, these are some of the techniques I use when I'm not relying on my massive bookmarks folder. There are others, but I think I've bored you long enough.
