Combo box associate key elements

My problem is creating a client side list of keys fields to associate with a combo box that I can access in the browser. General idea User selects one combo box and based on that selection, I want to retrieve a key from an associated array list in the client browser which was built at the same time the combo box was loaded. I will then use that key to go to the server and do a lookup. How can I store an in memory array of items which can be accessed in the client browser? If this was a win forms project, I would just create a static array list. With asp.net, do I need to create a hidden field or global session variable of keys for lookup without having to go back to the server for a selected index which takes too long.? A similar example would be where a combo box of states are selected like if Florida was chosen. I then use the item selected to find the key FL to go to the database table to load all the cities i ...