Most all wall/ceiling grills have
a seal on them from the factory.
But if yours are toast or gone
then the box store should have
some kind of 3/16" to 1/4"thick
mushy gasket seal that you would
find in there weather sealing
kinda dept. More inportant than
the seal between the grill and
ceiling though is the seal between
the register boot and the ceiling!
Buy some of the silver tape that
has the wax paper between each
layer of tape(the good stuff).
And put it from the inside of
the boot to the ceiling.The
condisation you are talking about
is most likely from outside
air being drawn in around
the gap between the boot
and wallboard.Good luck.
41Gasman