NI does not sell these screw terminals individually. I am not sure what third-party would sell compatible replacements either, unfortunately. If they are broken you could RMA them potentially, but that is likely to be more expensive than its worth. Wish I could be of more help.
There is no guarantee that it will fit but have a look at:-
MULTICOMP MC560-38115 TERMINAL BLOCK PLUGGABLE 15 POSITION, 24-16AWG
Unfortunately they do not have the screw fixings to hold in place