all are on line. And so long as over 50 you can apply. It's hard to open bank accounts without a visa and you need a Thai bank account for the 12 months extension of stay based on retirement. So get a visa before you arrive, it'll give you time to sort out the bank account.
All you need is to get your visa sorted before you leave USA. Enter Thailand. Open your bank account. Deposit the money. Then apply for the 12 month extension. No need for an agent.