Is it cheaper to book a round-trip or two one-way tickets?

For Thailand, a single round-trip ticket booked through a major carrier is almost always cheaper and offers better protection if your first flight is delayed.

Should I use a travel agent to find cheaper flights?

Generally, no. Online search engines like Google Flights have more access to real-time data than individual agents. Only use an agent if you are booking a complex multi-stop itinerary.

Finding Cheap Flights to Thailand

To get the best price, book your flight 3 to 4 months in advance and fly into Bangkok (BKK) rather than regional airports. Use Google Flights to track prices and set alerts for your specific travel dates to catch price drops.

Use the 'Explore' feature. Go to Google Flights and enter your home airport in the 'From' field. Leave the 'To' field blank or type 'Thailand' and click 'Explore'. This shows you a map of the cheapest dates to fly from your location.
Track prices with alerts. Once you have narrowed down your preferred dates, toggle the 'Track prices' switch on Google Flights. You will receive an email whenever the fare drops or rises significantly.
Look at alternative gateways. While BKK is usually cheapest, check flights into Don Mueang (DMK) if you are flying from within Asia. If coming from Europe or the US, compare BKK against flying into Singapore or Kuala Lumpur, then buying a separate low-cost carrier ticket to Bangkok.
Fly mid-week. Avoid Friday and Sunday departures. Tuesday and Wednesday flights are consistently $50–$150 cheaper on long-haul routes to Southeast Asia.

Is it cheaper to book a round-trip or two one-way tickets?: For Thailand, a single round-trip ticket booked through a major carrier is almost always cheaper and offers better protection if your first flight is delayed.
Should I use a travel agent to find cheaper flights?: Generally, no. Online search engines like Google Flights have more access to real-time data than individual agents. Only use an agent if you are booking a complex multi-stop itinerary.