sselec.com sells all sorts of replacement parts for battery packs. Take a look at what you have and then compare its specs with those available there. Thermistor resistors are readily available.
I took another look at the page I linked to above and realized that the specific part you are looking for is listed at the very bottom of the page in the right hand corner.
Mouser.com, Digikey.com, and many other electronics distributors. You'll need to know what you need though. Asking for a thermistor for a Rigid drill won't cut it with most electronics distributors, since they work based on mfg part numbers, not second or third tier assemblers (which RIGID and other companies are in reference to electronic parts).
Do you have the old part, and are the identification numerals and lettering still readable?
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http://www.sselec.com/data/Drill-pack-parts.htm
Do you have the old part, and are the identification numerals and lettering still readable?