What are the best options for booking a cheap onward flight from Thailand and ensuring a potential refund?

onwardflightsdotcom

Several options.

1. You just go to check-in, if they want to see an onward ticket, ask to speak to the supervisor and explain why you don't have an onward ticket. E.g. onward travel by land, changing visa status in Thailand, etc...

If they still insist, just book a cheap AirAsia flight there and then. Then cancel it within 24 hours.

2. You buy an expensive, fully refundable ticket directly by the airline. Then you cancel it once you arrived and get all your money back.

2. You buy cheap AirAsia to a neighbouring country and either visit the place for a few days, then come back overland.

3. Buy a booking reference on "Onward Ticket" or one of the many sites offering that service, but be aware, it's a scam. It might work, it might not.

Google “onward ticket”. You’ll find a plethora of companies offering you a ticket for $10-$13