George,

You can also buy them from the guys you bought your system from. Don't remember which one you got but either of these would do. I think it was the second one but all of the filter cartridges are the same standard 10 inch size.

http://stores.ebay.com/Filter-Direct...enameZl2QQtZkm

http://cgi.ebay.com/ws/eBayISAPI.dll...51000&rd=1