If you go to the Shaman guild and go through the teleport to the top, then go to east tower and go all the way to the bottom, then go to the pit and drop to the bottom there you will find the leeches. But be carefull the are other nasties as well )
Although it says these are in East Cabilis, I myself had a hard time finding them. As a matter of fact, I didn't even find them. Just head to Swamp of No Hope from East Cabilis. There's hordes of them there. Be careful though, cuz there's hordes of frogloks and mosquitos. And worse, kos range is horrid and all of them assist each other.
The scorpion, decaying skeleton, mosquitoes, a pygmy barracuda, a swamp leach, a giant swamp leach, a barracuda, and an Iksar outcast are all found in a secret tunnel in Cabilis East. I strongly recommend hunting there in the low levels, especially if you are a monk. You can bank and sell right outside this tunnel. The way you get in it is you jump into the water. Then you swim to the east end of the lake. You should find a tunnel there with barracudas and pygmy barracudas. In this tunnel you will find I believe 5 different places you can jump up and fight. Jump out of the water in the fourth one. Then follow a one way path till you reach the fifth one (you cannot jump into the fifth one for some reason). Here you should find the room. There is another secret room if you walk into one of the walls. When you want to sell/bank, just jump into the water and swim out.
They are used for a quest obtained from a guy in the monk guild. You fill the bag he gives you with 4 leech husks and Combine. Then give it to the guy in the shop across from the guild. (Same guy you get Curscale Armor from.