Assuming I have understood your description properly the part you want is called the diverta (no this is not a typo this is how Bissell spell it!) and is available from 4Bissell - just google it and they come up or ebay.
The best I can find fro schematics is here http://www.hescoinc.com/vacuum_cleaner_parts/bissell.html
But fitting the diverta is not too hard - you have to take off the top deck and get at it from underneath.
Chay, October 2009