You may have a problem leaving your departure airport because Thailand requires a return or onward ticket out of the country in order to board the plane. I will say it depends on the check-in agent at the airline. My last time flying into the country I was able to check-in after showing my current lease that I live in Thailand. Another option that I have used for several countries is
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There are similar companies that provide the same service but I can only speak about this company. You purchase a ticket through them for $14 or $16. The ticket expires after your flight. I have never had a problem doing this.