USA Triathlon has a page devoted to this topic on their website at: http://www.usatriathlon.org/audience/coaching/become-a-coach.aspx
Explore their site; it appears full of good information. Best of luck.
I got a NASM certification a few years ago. I checked at a few gyms beforehand and they all accepted NASM. I thought the study program was pretty good and the cost of the program was competitive with those offered by others. Study the materials they send you for about 2 to 3 hours a day for a month or so and you should be good to pass the test. Good luck and have fun!