This is a really old post but I found this article really helpful:

http://www.codeproject.com/Articles/3628/Letter-Alphanumeric-Based-Paging-in-ASP-NET

I used the UsersViewer.aspx and its attached UsersFilterControl.ascx

I created a public property in the Filter to pass the Users data source to the filter

  public object UserDataSource
    {
        get
        {
            return mDataSource;
        }
        set
        {
            mDataSource = value;
        }
    }

And passed it to the filter control (about line 720 of UsersViewer.aspx - note do this after the initial WhereCondition has been applied to the UsersDataSource):

filterUsers.UserDataSource = srcUsers.DataSource;

In the letters_Bind() method from the above article I changed the binding so that it only shows the letters from users that we have in our datasource.

 InfoDataSet<UserInfo> uds = (InfoDataSet<UserInfo>)this.UserDataSource;
            Hashtable htUsers = new Hashtable();

            foreach (UserInfo dr in uds)
            {
                string firstLetter = dr.LastName.Substring(0, 1).ToUpper();

                if (!htUsers.ContainsKey(firstLetter))
                {
                    htUsers.Add(firstLetter, firstLetter);
                }
            } 

In the filter I changed the letters_ItemCommand (from the above article) to the below:

if (_letterFilter == "All")
            {
                WhereCondition = "";
            }
            else
            {
                WhereCondition = "Upper(LastName) like '" + _letterFilter + "%'";
     }
   
            // Raise OnFilterChange event
RaiseOnFilterChanged();