There Is A Fungus Among Us.

Vanuatu
Fungi
January 9i, 1984
The stamp set features Cymatoderma elegans var. lamellatum, Lignosus rhinocerus, Stereum ostrea, Ganoderma boninense.

Poland mushrooms

Here we come with another nice issue!
It is from Bosnia and Herzegovina (Muslim part).
It come to the market in 1998 year, set of 4 values, nice design, and also affordable catalog value of 4.50 euro, per set in mnh condition.
Listed in Michel catalog with numbers 141-144.
Probability that this set is still available on market exist, because it was printed in 50 000 sets.

Just to "update" post from Danijela:

Issue from Bosnia and Herzegovina (Croatian part), is one of better made in this 2010 year, it come as set of 2 values, with Michel catalog listing
numbers 303-304, again affordable catalog value of 5.00 euro per set in mnh condition.
Probably available on market, but less frequently, as printing quantity are just
25 000 sets, keep it just as "pair" in album and not separately.

This set is issued in sheet of 5 sets with margins, which give us better to see the beauty of this issue!

Bulgaria
Mushrooms
November 17, 2004
The stamp set features Polyporus squamosus, Fomes fomentarius, Piptoporus betulinus, Laetiporus sulfureus.

Greenland Edible Mushrooms
2005: Leccinum sp., Russala subrubens, Amanita groenlandica
2006: Rozites caperatus, Lactarius dryadophilus, Calvetia creatacea

European Poisonous Mushrooms, issued by German Democratic Republic (DDR, East Germany) on March 19, 1974.