As far as I can tell, this filter is not being made anymore. I have not been able to find a source and the company I found that had them was out of stock and didn't know if they would be getting anymore.
Otherwise, I would recommend replacing your entire countertop system with one of the following models:
Countertop water filter systems and counter top filters from Culligan, Ametek, USFilter, and American Plumber
The QMP602, QMP603, and the Digipure 9000S SA168 all use approx. 10" L x 2.5" W cartridges. These cartidges are standard size and available from many manufacturers so you won't be in danger of them being discontinued again.
Thank you,
WaterFilters.NET