Purchase directly from the major insurance companies, specifically through their Thailand branch. They can provide a quote for 6month or 12 month.
They cater to expats & retires.
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Top 10:
AIA Thailand
Muang Thai Life Assurance
Allianz Ayudhya
AXA Thailand
Bupa Health Insurance (Aetna Thailand)
Cigna Thailand
Thai Life Insurance
Krungthai-AXA Life Insurance
Generali Thailand
Pacific Cross Health Insurance