if you can find out what kind of temperature probe the manufacturer has used, if there is a product number on the probe, you can try to get the right spare part cheaply:

www.digikey.com <-- for manufacturers and electronics engineers and electronics prototyping supplies, they have thousands of temp probes. yep, thousands. you just need to find one that is the same, or that is exactly equivalent.

apart from that, different companies use different temperature probes from different suppliers and with different characteristics. so you cannot just buy a temp probe from any supplier, it needs to be exactly the right one, compatible with your thermostat. so that one from helix for 20 dollars that freaky_frog suggests may not be the right one. the supplyer of your thermostat is the first adress to go to, hopefully they have the right spare part, or can at least tell you the correct technical data on the probe. with the right data, you can again try digikey.