There are two possibilites for plunge freezing in LN2: one is “normal” liquid nitrogen -- as it comes from the dewar, the other is slush nitrogen.

If using liquid nitrogen without slush, then something like isopentane, ethane, or propane (ordinary cooking propane will do) must be used. The caveat is that using these liquid gases is a serious fire and explosion hazard, especially since liquid oxygen forms at liquid nitrogen temperatures, and dissolves into the liquid hydrocarbon. These gases can be used safely (1 have done so) but they take care and understanding of what's happening, and a safe place to dispose of the cryogen.