The best way of finding a good TA is personal recommendation. I deal with a very good one, but I don't believe specific names are allowed here. Next best way is to look at the agents who advertise on this board and the comments related to them posted here as well.
Look at credentials, experience and "interview" a potential agent. Do not (in my humble opinion) deal with someone who is a "price quoter on the other end of an email".
I want someone I can get face to face with, who asks me questions and knows what I like. I want someone who is also a cruiser and "talks" cruising with others. Someone who knows what is going on in the industry.
I do not agree it is necessary to be brand loyal to a single cruise line. I have sailed on 5
in 13 cruises (with 14 coming up next week). Each had an advantage of ship, destination etc.
Do your own research and ask your own questions. Be knowledgable consumer, you are spending a lot of money on a cruise.