I have a JD 717A and bought blades from:
They are in IL, and, from the website, look like they are heavily into the ag business. However, I called them, placed an order quickly for exactly what I wanted, and the shipment arrived in a couple of days. The person on the phone knew exactly what I wanted. I think they sell Ztrak items on e-bay. I don't buy off e-bay, so just called direct.