If you are focused on just a few hub airports, you might find something like https://direct-flights.com sufficient. It shows scheduled departing flights and their destination for a given airport. Source: over 1 year ago

If you're on a one way ticket, the airline should ask for your onward ticket details before you board. Immigration almost never does since they assume the airline did and it would be difficult for them to be able to check if a ticket is valid. The onwardticket.com is a good option. Airlines ask since if you're refused entry they need to fly you out. Source: 6 months ago

You'd need a return ticket within the initial 30-day window a visa exempt entry grants. You can do this with a rental ticket (onwardticket.com) or with a throwaway cheap ticket (which aren't as cheap as they used to be). As far as the felonies go, I think it would depend on what they are. If they were serious enough to put you on a no-fly list, sex offender list, on parole, or similar then the airline and Thailand... Source: 6 months ago

Second: Sites like onwardticket.com say they’re providing you with reservations but a site like BestOnwardTicket says they book you an actual ticket. Is there a difference between getting a reservation or getting a ticket when it comes to providing proof of onward travel (as opposed to providing travel and hotel info for a visa application)? Can it just be a reservation or does it have to be an actual booked and... Source: 6 months ago

I've used onwardticket.com several times, always with success. Source: 6 months ago

Onward site https://onwardticket.com/ wants 16 usd for a real flight ticket. Source: 6 months ago