Finding dungeons is all about luck, I've found 2 on my world so far. The best way to find them is through cave exploration on Easy+ difficulty, this way so that you hear/see when there are large amounts of the same type of mob. That will sometimes indicate a nearby dungeon. Playing on peaceful makes dungeon finding twice as difficult :/
The sig you are trying to read has derped itself. Please come herp again later. I can kill people with safety scissors and pins. Ask me anything.
I've found all three spawner types in my world and a couple more in my friend's worlds. Listen closely for mobs on easy or higher. Sometimes you can find a spawner just by following the sounds, especially if you can hear several mobs of the same type.