For most countries these days and onward ticket to anywhere is required within the 30 days or however long the visa is. It's not always immigration but the airlines because I have been asked in the past year going into Thailand and I am a retiree from US.
Rent one from
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for 10 to $15usd and you'll receive it within an hour and just choose a popular city so they'll be lots of choices to reserve you a temporary flight and then they will cancel it within the day or so after it's date. Saves you the hassle and waiting for a refund.
Whatever company you use, be sure they say it has an actual PRN number which is your booking number so that if anyone in the rare event checked, it would show as legitimate. This company does and so do some others. But do print ticket separately so no one sees the email that it comes from or download it on your phone as a separate document.
Avoid burning season in Northern Thailand and other parts of Asia:
March through May?