Extremely rare and many games two simply will not generate, even on Voyageur or Pilgrim, though one almost always does even on Stalker somewhere in the world (but never Interloper).
While they are warmer than a Fisherman's Sweater, they weigh twice as much (1.0kg VS 0.5kg) and -1% slower, for only 0.5 degree warmth/0.5 degree windchill better. Fisherman's Sweaters are also far more common, with 8-10 spawning regularly across a Voyageur world.