if you need a visa, then you must get it before you arrive. Otherwise you'll get a visa exempt stamp for 60 days when you arrive (assuming you have American passport?). Once you're here, you can sort out a longer-term visa option or search for a visa agency.
Application fee is 10,000 baht. And it's very easy to do yourself, there's no need for a visa agent for DTV. This sounds like a total rip off and possible scam.
What do you mean by don't trust minivans? This is the best solution for you to achieve your stated goal. Quick border bounce through agent will guarantee your next visa exempt entry of 60 days. Then you can extend it for 30 more.
You may want to consider getting the criminal conviction removed/expunged from your record. The waiting period prior to application will depend on your country.