I just picked up a Jet 16 drill/mil
and its missing
lower leadscrew nut
and both gib plates
This is the "Taig Mills & Lathes" forum. It is dedicated to a certain brand (Taig) of mills and lathes. You'll probably get better answers on the more general "Benchtop Machines" forum.
It's pretty likely Jet isn't the only manufacturer or that mill/drill so if you can't find an "official" Jet parts source you can probably get them from somewhere else.