Yes
Arrival card ( TDAC ) needs to be completed within 72 hours of arriving into Thailand
Skyscanner or webjet will give you the all the flights coming over
Pick the the most suitable & book direct with the airline itself ( most likely Jetstar or Scoot )
If you have t. Even I Thailand in the last 6-12 months , you probably won’t be asked to show a return ticket , however if you want to have something to show
Use Onward ticket
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Or get a bus ticket from
Udon Thani to Vientiane ( Laos )