If its not the part I linked then its PROBABLY a part that comes with the PDC front bumper instead. Meaning, when you buy a front bumper with the PDC option it comes with the sensor holders.

If you cannot purchase it separately then you'll have to pull one from a salvage yard or get lucky on fleabay, CL, etc. Its hard to believe you couldn't get that part separately but you never know.

Next time I take my bumper off I'll have to take pics for future reference.