In few words…
Character: you need a lvl48-52 character. Mercenary with DEX build or a Mage FullINT preferred.
Where: Between Darkon and Flaris.
Killing: Captain Rockepellers, flying monsters, lvl50.
Why: CptRockepellers has got a HUGE possibility of dropping a PLUG+9.

The monster:

Captain Rockepellers. Flying in the low air on the path between Flaris and Darkon, but also in the high air just a bit south of it. These are high-HP monsters, totally useless to kill, if it wasn’t for the plugs….
These monsters are electrical, so they’ll suffer damage from +earth weapons. That’s why I suggest refining all the equip with +earth.

Here’s the drop rate of all Rockepellers (thanks to ItsReiko):
Small Rockepeller drops:
DemolEarring+3 – 0.4% chance

Normal Rockepeller drops:
PlugEarring+9 – 0.4% chance

Captain Rockepeller drops:
DemolEarring+3 – 0.4% chance
PlugEarring+9 – 0.4% chance

Giant Rockepeller drops:
DemolEarring+3 – 1.2% chance
PlugEarring+9 – 1.2% chance

The drop:
Basically you’ll drop two items: DemolEarring+3 and PlugEarring+9.
First one is a version of the Mighteer, but it’s +attack rate is not even noticeable while you kill. In fact, this item is almost worthless, in front of a Plug. The second one is what you’re going to drop. The PlugEarring+9 has got almost the same devense of a Penzeru+1(now PlugEarring+10), but is way cheaper to drop, or buy. The +def is +96, incredible for any AOE class or any low STA class needing of a huge defense. Becouse two plugs+9 @ level50 can add a +50% of defense or more very easily.

The character:
There are two types of characters to go dropping plugs.

Easy way: level 50 mage PureINT. Get one, no matter how. The equip is, obviously, Teba Set+3+3earth with Branch Of Rhisis+3+3earth. The pros of using a mage is the easy way of killing, since you just have to target and fire, you’ll launch “homing missles” to the monster, damaging pretty high. It’s good for a computer suffering of lag, becouse spawning and vanishing monsters will be automaticly re-targeted and keep being killed. Still, it’s not the fastest kind of character to kill.

Hard way: level 50 mercenary Dex Blade build. This time equip is Pangrill+3+3earth and Masquerpet Axe+3+3earth. You need master Axe skills for this, so you’ll hit a lot more. In fact, mercenary is way faster at killing Rockepellers, but you’ll have to run after the monster and catch it. This means that a lagging computer won’t be adapt to such character, since when the monster will vanish cause you’re lagging, you’ll lose the target, and start running after it again. But if you have a good connection, and you’re not going to watch TV in the meanwhile, this is a great killer.

The party-more luck-whatever:
Actually, I have never seen any difference between being in a party or not, having cheer or not… and as usual I only think these are only metropolitan legends made by noobs. I think the best way of killing these is just going there when you don’t have enough time to go level and so you can’t do nothing else. You don’t need a party or nothing. Just put your killer between the monsters, and go there whenever you have a minute to lose. The drop rate is random, so also the first monster can drop you a Plug.
Partyes aren’t of any help, at least for my experience. Neither GB or FC. Still, this is not an accurate information, so it can be just an impression.

Appendix A: from v11 and on…
Yes, plugs+9 still drops, it’s proven. But this is not really the point. Now you can use all the jewels and reductions, so the plugger life will become sweet. We’re now able to use strentes+1 and mighteers+1 (called now vigors+9 and demol+12) and get a dretra sword to be reduced, getting way higher damage and aspd.
Awakenings can help too, but I suggest to just use normal green equip.

Now learn fighting in the air:
1)Click two times on the flying item icon to jump on the board/broom.
2)Get at Rockepellers.
3)Use TAB to select the monster, keeping it in the middle of the screen.
4)Press Z to start homing the monster.
5)Press INSERT and while you keep INSERT pressed press on ENTER. This will activate the AUTO-ATTACK.
6)If you’re a mage, you should be already sending spells to the monsters. If you’re a mercenary, you must touch the monster to start killing.
7)Press SPACE to stop, so you don’t get dizzy while killing.

Repeat from 3 infinite times, but skip 5, becouse is already activated.

And now?
Drop 2Plugs+9 fo yourself. You’ll find them useful forever. Then start dropping some “for others” and sell.

Good luck. This ancient guide still helps