Where is the cheapest place to get PortAventura tickets online?

I'm going to Salou in 4 days and I am planning to go to PortAventura with my family. I just want either a 1 or 2 day ticket for 2 adults and 2 children (18 and 12). And how much is the express ticket thing where you don't have to wait in queues? Thanks in advance :)