Combined ticket online to Borobudur and Prambanan temples

Good morning,

Who knows how I can buy the combined ticket for Borobudur and Prambanan temples? I have already tried the official website, and I can not make the purchase. The site takes a long time to work and then leaves without giving me the proof of purchase of tickets with visa card.

Is there another website where I can buy the tickets?

As I arrive in Borobudur in the day before I want to make the visit to the temples I am afraid of not getting tickets.