If you have to buy an onward ticket out of Thailand, just book a full fare business class ticket to a nearby destination, and pay it with your credit card. As soon as you arrive in Bangkok, cancel it, and if it is a full fare / no restrictions ticket, I believe you can get a full refund from the airline. Pay it with your credit card, and depending on your billing cycle, the refund will appear on the same statement as the charge for the ticket. I did it once, and it worked.