I needed to show a proof of onward travelling to board my plane to Colombia from Panama. I used the service of Flyonward.com to give me a real legit ticket for 24h. So for $10, they book you a flight on a real international flight (at random) out of your country necessitating the onward ticket (in my case, it was Colombia). I got my plane ticket 10 minutes later. No headache. And way more comfortable than faking an onward ticket (imagine getting caught at the gate...)