Do Thai people need visa to Cambodia?

Yes, but it is a Visa-on-Arrival. At any border crossing, airport, or seaport of Cambodia, after crossing the Cambodian border, you will pay a fee of between $20-$25 USD for a Cambodian 30-day travel visa. Do not buy a Cambodian VISA from any person, group, or vendor outside of Cambodian sovereign territory as these will generally be overpriced scams.