Incomming wall of text.
Some things to consider - most people who "plex" find their plexs through exploration. There are static plexs out there, but as you've discovered they're horribly farmed 23/7.
The ones you stand the best chance of getting (there is competition for these too, but since you have to find them first and they despawn once completed it's MUCH more likely to get one that isn't being run) are a type of Cosmic Signature (not Cosmic Anomaly). When you resolve them down to 25% signal strength, they will display "Unknown" as their type (the same as wormholes), and when you get to 75% they will display a name (Unstable Wormholes obviously aren't a plex, and something that I won't go into here).
There are 2 types of plex you can find, and they can be in any security space (but obviously the best/hardest are in 0.0), the DED rated complexs (what you call deadspace complex) are the hidden counterparts of the "static" ones you find on those beacons, the ones being farmed to death. These will display a rating out of 10 when you warp to them, indicating their difficulty/ship limit. 3/10s allow cruisers, 4/10s allow BCs and 5/10s allow battleships. At the end of these complexs (usually several pockets), there will be an overseer who has a chance to drop faction/deadspace loot. This is where the ISK is.
The second type is what I call the unrated plex. It won't display any type of rating when you warp in, and the only warning you'll get is if your ship is too big to go through the gate. Pay attention to the messages you get when using gates, and any text in local - it will generally show you what you need to do to advance to the next area. In the last pocket, there will be some trigger (indicated by the flavor text) that, on it's destruction, will give you a chance for two things to happen: first, a faction commander may spawn (i.e. True Sansha Ship Name or Dread Gurista Ship Name), the second thing that may occour is you may get a popup along the lines of "Your ship computer intercepted some coordinates!", this is called an expedition (or escalation) and can lead you to another hidden area where you will encounter more enemies, a faction commander, and possibly a further set of coordinates. Expeditions (escalations) have their own section of the journal that you can use to view any ongoing ones. You have 24 hours to complete each stage.
At each stage, unless something unusual happens (I can think of maybe 3 or 4 times), there will be a faction commander of the appropriate size who drops faction loot. It may be the dreaded "tag and ammo", which let's face it, is junk, or it may be something a little nicer like a pirate implant (low-grade crystals and slaves can be worth 100m a piece, you can find these in high-sec) or a faction or deadspace module (for instance Domination 10MN MicroWarp Drive or Centii A-Type Small Armor Repairer, or if you get unlucky yet another True Sansha Small EMP Smartbomb).
Exploration (well, outside of wormholes) is entirely based on the wonderfull Random Number Generator game mechanic - you can get nothing, or get loaded, in no time at all, or days and days. I wouldn't reccomend it as a way of making a "stable" income (like, for instance, running level 4 missions). But I found it a lot more exciting, and it can be much more rewarding if you get lucky.