I am trying to buy entry tickets from the official website http://www.machupicchu.gob.pe/ but it never seems to accept my Credit Card. I tried 3 different VISA cards.
Is there any other online alternative?
Update: Good news, when you load the official page, you'll now see that they accept VISA payments. As a result, you can now buy advance tickets from outside of Peru directly with the website.
However, there are still many other ways to do this. I'd recommend this guide for sources on how to do it.
There are numerous online tour agencies that you can pay to get you tickets.
Once you're in Cusco, you can buy a tour then if you're doing one of the trails.
Once you're in Aguas Calientes (final stop on the train route), you can buy tickets there.
You can even buy tickets at the last second at the gate when you arrive at Machu Picchu.
So don't despair, there are plenty of other ways to achive your dream of going :)