I haven't found a place that I can cultivate in the Mistlands.
BUT someone in thread yesterday mentioned planting
them on the edge between Black Forest and Mistlands...

that works: cultivate along the edge and plant them on the
dividing line. One mushroom makes 3. No seeds.

I think there must be some better way though...
in the detailed patch notes:

Mushroom farming (The new Mistlands mushrooms can be grown from the comfort of your own home)