You can buy PortAventura Express from the ticket windows, the Express booth just inside Mediterranea or the vending machines rather ironically located next to the queue time boards for each of the rides that have an Express entrance. Unlike Alton Towers though, you have to buy one ticket that includes 10 rides rather than being able to select individual rides. You can also get an unlimited version too.
If you want to sit in the front row of Shambhala or Dragon Khan you will probably need to buy the VIP add-on as they rarely allocate the front row to people that haven't paid for it!