Target has a 15% off Cartwheel discount on Nest Home Products. With the recent release of the Nest Thermostat Sensor which gives the Nest Thermostat the same functionality as the Ecobee, this is a good deal for Nest Thermostat Gen 3 owners.
$39 - 15% discount = $33.15 + taxes. Save an additional 5% for Redcard holders = $31.49 + taxes.
Best Buy sells a 3-pack for $100 but you have to buy 3. This is good if you only need 1 or 2 sensors.
Nest did send a $20 coupon for the sensors to select owners. I believe that Nest sent the discounts to owners that may have had the Gen 1 or 2 thermostats to incentivize an upgrade to Gen 3. If Nest did not send you one, like me, then this might be the best deal for the sensors right now if you really need one.