I have found that if you go to local office of all these Government offices and ask them direct question, what do I need to provide so you can issue me with what I require, and then you go away and get what they said, I have never had an issue.
You need to do this coz every office in Thailand decides for themselves what they require.
Most of the time not the same as what you read on Government websites.
Have never used an agent.
Agents cost money and tend to be used by those who cannot provide what is needed.