Hi,
Try
http://www.vernier.com/probes/ for all the probes you could ever wish for!
Not a bad price either. You will need some sort of gain circuit or a/d conversion to make them work on PLC's. Also you will need to poll some of those probes one at a time and take a reading as some of the probes conduct a small current through the water and if 2 are operating at the same time, you will get erroneous readings.
Polling should be a sinch with a PLC though.
Hope this helps!
VR
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