From the limb clinic head left then there will be a gas station on the left that has one of the arms dealers in it, from here exit the dealers store go left around the building into the alley. Follow this alleyway to the basketball court, across the alley from the basketball court will be a ladder leading to a fire escape, climb the ladder and look at the windows as you go up. You will find one that you can open follow the hallway to the last door on the right and you will find the second dealer.

Uh, yah. What he said. Finally found it through exploration an this guy got this sh'izzle down.

There is also another dealer as you get further into the game he's in a sewer