Big area for camper vans on the main road side of the river, right by the paddock, see their website. http://www.mototrial.it/2013/Gare-2013/MONDIALE/Barzio/info-barzio/Mondiale%20trial%202013%20barzio%20informazioni.htm
You can buy tickets as you enter the area around Barzio, no problems at all. You can also buy a car pass that lets you park at the groups if you have a hire car. Saturday is always good for checking out viewing spots during the Euro round. Good hotels in and around the area. Good group of sections below the cable car Andy mentions.