As I recall, there are four defrost "boots" under the dash, two on each side of the radio speaker grille. When I pulled mine out, I couldn't re-use those gaskets either, so I got a roll of 3M fingertip caulk.
It's not like a gel caulk that comes out of a caulking gun. This stuff peels off a roll, is pliable and squeezable. Pull off as much as you need and cut off. Lay the caulk in the little recess provided where the gasket was and smash the caulk down just by tightening the two nuts on the ends when installing. It makes a good permanent seal.
You should be able to find 3M fingertip caulk at some of the more premier hardware stores. It comes in a square box.
