Hello there! You may have figured this out by now, but just in case Black mushrooms and Green mushrooms are common hoard items, which means they have no color associated with them and can be found by any Pixpet. ^^ The same goes for Liquid items (Pearl, Sapphire, etc.) seeds, special ingredients, and all other ingredients in general. (With the exception of Pumpkin-related products)

The chances are pretty high for finding them, happy hunting!