How do you implement a combobox filter using AJAX in ASP.NET?

Question

To save some time on discussing my problem you could check the demo below:

http://demos.telerik.com/aspnet-ajax/combobox/examples/functionality/filteringcombo/defaultcs.aspx

I already checked the ListBoxExtender on the Ajax Control Toolkit but it wouldn't give me fine results.

What I want to do is to filter a listbox which is populated by over 3000 records from the database upon typing. It should not only filter the listbox with the starting letters but also the group of characters which could be found in between each item on the list.

The list is a list of Item Name as a value and an Item Code as the key.