order object and sessions

Hi 
I have a aspx page called ProductList, this page have a ListView which lists many items. 
For every row/itemtemplate there is a button. If the user clicks this button all the values relating to that row (price, product name, product nr. etc) are inserted into a ItemObject (The item object just comes from a item class with corresponding properties: price, product name etc.).
Note: All of the above is done within the Button Click event in code behind.
At last I want to add this itemObject to a Order object (this is also done within the button click event).
The order object also comes from a Order class which contains an ArrayList that takes Item object. The purpose of this Order object is to have it contain all the items object the user has selected. Later on when the user wants to 'check out' I will show him all the items within this Order object. 


Now my question is where should I instantiate my Ord ...