I second the suggestion to use Trainline.eu to buy tickets, as they sell tickets for a variety of rail networks. You'll have to register on the site first, including payment details, but once that's done the process is very user friendly.
You don't need to buy advance tickets for the Milano Centrale-Malpensa route as that's a Regionale train and prices are always the same. You can buy tickets on the day of travel or the day prior to travel at local stations. BE SURE to specify the correct date of travel when you buy tickets. Also, you MUST validate locally purchased tickets prior to boarding the train on the day of travel, or you'll risk hefty fines which will be collected on the spot. That apples to all Regionale trains, buses, metro, etc.