the govt created an insurance policy/portal just for foreigners coming in, to meet this requirement. I used it and it's super easy, takes 5 minutes and the price depends on your length of stay. They immediately send you the policy via email on completion of payment.
I used the Thai govt Covid Insurance for Foreigners portal. They have a group of 16 insurers. It's very, very easy and the price depends on the length of coverage you want. Takes 5 minutes online, they email you your coverage letter and policy immediately...finished.
I used the Thai Government travel insurance for foreigners portal created specifically for this. It's a group of 16 Thai insurance companies that have agreed to offer Covid-19-only insurance at $100k coverage. This fulfilled the requirement for me. The price is by duration of stay, the whole thing takes less than 5 minutes and is very straight forward.
It's ok with my to use agents, they're firmly baked into the system in Vietnam and I've been using them for years. Do you have to use an agent from the area that you're staying in? So if I was in Bangkok, I'd have to use an agent there?