Closest city to the border for buying an onward ticket to Panama

Hi everyone, I’m planning to go to Panama in December and I’d like to know which city is closest to the Panama-Costa Rica border—and just across the border in Costa Rica—to buy the cheapest bus ticket? Just so I have proof of onward travel when I take my flight, thanks.

Easier option: go to the site onwardticket or onewayticket, you buy a ticket valid for 48 hours, it costs $16, and at least it's safer than the bus, which isn't accepted as proof of exit everywhere.

Thanks.

I know there are ticket-selling sites like safeonward.com, but I don’t know if they’re any good.