Yes I’ve dealt with changing many desk top fans and PUC in the past so have some experience. Is it just the one size fan? Is there a link perhaps I can buy one?
My Dahua one has a 40mm*40mm*10mm fan and the power supply has a 40mm*40mm*20mm server style fan.
Small one is 5v and the power supply one is 12v. 2 wire fans.
You're best to open the NVR up and see the motherboard fan. With the power off [and with caution]; open the unit, also the power supply module to see it's fan.
Take note of the size and voltage [& current].
The replacement fan should be of the same voltage but no higher than the current on the previous fan (There is no easy way to tell the maximum current the fan can draw.)
I have no idea what your Hikvision NVR's fan size is. For all I know; it may not have a fan in the power supply.
Remember to treat the power supply circuit as if it's live; the capacitors inside can deliver a shock even when the unit has been switched off for a while.