I usually go through Amazon or direct from ThermoWorks and/or Maverick....
I ordered a few weeks ago from
http://www.bbqequipmentstore.com/Replacement-Probes_c169.htm
If you order two or more and they're $10 each. I got a pit probe and a food probe. But they seem to be short as well -- only ET-72 food probes in stock.
The 72/73's are sold out just about everywhere. I just checked Maverick's site and they are sold out. I would love to use the Thermoworks probes, but they just don't work correct when I use them like the 72/73.