In most cases, the landlord pays the agent's fee, not the tenant. This is standard practice for long-term residential rentals in Jomtien -- the agent is typically engaged by the landlord to find a suitable tenant, and their fee is usually calculated as a percentage of the first month's rent or a flat amount, paid once the lease is signed.
As a tenant, this means you can generally use an agent to help find and view rentals without expecting to pay for that service directly. Your upfront costs are the same either way: security deposit and advance rent.
There are exceptions -- some arrangements, particularly for harder-to-fill units or very short leases, may involve a tenant-side fee, but this should always be disclosed clearly before you commit to anything, not added as a surprise at signing. If an agent asks you to pay a fee, it's reasonable to ask why, since it's not the norm for standard long-term rentals in the area.