Mike,
I have the Jet JJ-6CSX jointer.
I've ordered Freud knives from Amazon. They are on the jointer now and seem to be okay but are a tad shorter than the Jet originals. I have also ordered from Global Tooling. They seem to be as good as the others and are less expensive. They are the same size as the Jet originals.
http://globaltooling.bizhosting.com/planer_knives.html
Tom