Lulzbot doesn’t seem interested in selling a lot of parts. But, there are other companies who apparently see that as an opportunity… I think this might be the thermistor you’re looking for: https://itworks3d.com/product/thermistor-3d-printer/
[Edited to add: they seem to be out of stock as well… e3d-online claims to have them, but you’ll have to get them shipped from the UK… ]
LulzBot does have Marlin sources with config files available for all of their toolheads, past and present. Knowing which Marlin thermistor model is currently used should help you find one that uses the same algorithm. The firmware has the ability to adjust the PID values.