I didn't use an agent when I applied for Residency, and all up it cost me about $1500 to become a permanent resident. You can probably find agencies/agents through a search on the internet, but how you'd learn about their credentials, I'm not sure. I know that when I went to the Immigration office in Auckland (I arrived at about 4:30am and was behind 5 other people) there was a professional place-saver sleeping up at the front of the line, who was paid off by the agent when he arrived just as the doors opened at 8:30am. Part of your fee might go to contracting out services like that.