You don't really need the thermal sensor, my iMac 27" doesn't have one connected to its HDD because it has an SSD which is incompatible with the jumpers that the sensor uses. Basically, it is generally okay if its not connected. The computer will think its 128° C and the fans will run mad. If your okay with the sound of the fan (which I am) then you can keep it like that.
If you insist on a thermal sensor;
Apple iMac 20" Thermal Sensor Optical Cable 593-0191-A
Apple Macbook Pro 15" Middle Thermal Sensor for A1260
I couldn't find any specifically for your mac mini, however I do believe they are (somewhat) interchangeable
General search for thermal sensor
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