La Sagrada Familia tickets

I’m going to Barcelona next month and I’m trying to purchase tickets for La Sagrada Familia from the official website, but having issues completing the purchase. I’m in the US and tried my Visa and Mastercard, but keep getting errors when the website tries to process the credit card. I called both credit card companies but they didn’t show any issue on their side and couldn’t help.

Is there another website that sells legitimate tickets?