If you check the parts manual (TM9-2320-280-24P-1), you'll find the coolant temperature sending unit is item 11 in figure 66. That will tell you the correct part number and NSN so you can check around at the various military parts suppliers to find it. KASCAR, Erik's Military and Industrial Surplus, Eastern Surplus, Memphis Equipment, Oshkosh Equipment, CSI, Inc, all carry it.
You can also check the CUCV parts manual and see if it is the same sensor, although it's probably not since the majority of the electrical system (outside of the starting system and radio power) on those is 12V while everything is 24V on the HMMWV.
All of the manuals are available for download here:
https://www.steelsoldiers.com/forumdisplay.php?95-Technical-Manuals.
-Ken