How can I find a German-speaking agency to help with the DTV Visa in Thailand?

Yes:

@Stefan *******

can help you 👍🏼

Honestly you should be able to give it a go yourself. Get current bank statement showing proof of required funds, imsge of your passport, selfie on your phone of head and shoulders, contract (which you can get AI to do, include "required to work remotely", put in the HR or Directors name, sign it yourself in their name, have the contract addressed to Thai Embassy, have relevant flight number that you'd like to use to fly into Thailand eg QR19. Get utility bill or something from govt, like a tax return with your name and address on it. Really really easy. I did it last weekend via Embassy in London. Sorry can't help with agent.

Why does that person need to speak German? You just give her your money and passport.