You can also farm Fumes in Garmet District. He's underground, so for those of you that don't have any other bosses, there's a good chance you haven't killed this one yet:
When you spawn in the safehouse, move out the ground sewer exit in front of you and turn left.
Climb up the two ladders and proceed to follow the only path out.
As you're running along, pay attention to the left as there should be an open door with a hallway leading to the subways. (You should have only climbed two ladders at this point, if you climbed three you've gone too far).
Once you reach the subway, turn right and run along the wall.
There won't be anyone there initially, but Fumes (wearing all white, hard to mistake) and his minions will spawn out of thin air in front of you.
Kill him, and let his minions kill you (shouldn't take long, they all have flamethrowers).
He drops two purples and up to 3 phoenix credits from what I've seen so far.