Mushrooms can be bought from the mushroom dealer. Click on "Mushrooms" in the game options, select your mode of payment and there you go.

There are also "Offers" for free mushrooms. Complete a survey or play a game to gain the mushrooms. In case of problems please click the support link under each offer.

In addition, you can find mushrooms in quests. You do not know if you will find a mushroom when you start a new quest but if you are lucky one might appear when the quest is completed. Without a mount the chance to find a mushrooms is 8%, with a Griffin it is only 5%...which means that the faster you travel, the less likely you stumple upon a mushroom.

Moreover, you can visit the gambler in the tavern. Note: you can only play for mushrooms if you already bought mushrooms from the mushroom dealer.