ComboBox items loaded on demand using the ItemsRequested event handler do not exist on the server and cannot be accessed using the server-side FindItemByText / Value methods.
This is needed for speed (otherwise the combobox will not be that responsive upon each keypress if state information and ViewState were persisted).
Also, the eventArgs.NumberOfItems is only used when ShowMoreResultsBox mode is used (to calculate the message in the results box) and cannot be used in standard load on demand mode.