Took a gamble Amazon and found a workable one:
https://www.amazon.com/dp/B07LGMXTKK
Description:
== uxcell 2 Pcs 10K NTC Thermistor Probe 15.7 Inch Epoxy Sensitive Temperature Temp Sensor for Air Conditioner ==
I think the main thing is just finding a 10K NTC thermistor that is waterproof (thus Epoxy in this case).
After putting it in, water temperature was +/- 1 degree off what my thermapen registered. Not bad for $5.49 for two of them!