Hi Satha,
Very difficult to get D. decandra seeds ATM as it is out of season. Even if managed to get them, most of the time they won't germinate as the seed of the species has a very short viable period. The only way is to get a grafted plant from Thailand but the cost is high, roughly US$70 plus phytosanitary certificate around $20+ and EMS shipping is an extra. There is also no guarantee how long it will take to reach you as currently the global postal service is totally unreliable due to the pandemic.
I don't think importing plants to Sri Lanka is easy, as far as I know.
Regards,
Paul