The correct preposition to use with "champignons" (mushrooms) in French is de. This is because "champignons" is a plural noun, and "de" is the preposition used for partitive articles (some, any) with plural nouns.
Here are some examples:
* J'aimerais acheter de champignons. (I'd like to buy some mushrooms.)
* Il y a de champignons dans le jardin. (There are some mushrooms in the garden.)
* Elle a mangé de champignons. (She ate some mushrooms.)