I used a dried mushroom blend, which is a great option instead of buying each variety separately. Otherwise, you can find the dried mushrooms at specialty grocery stores in the produce section. And for fresh mushrooms, I used cremini and shiitakes.
From: Bon Appetit (12/09)
Bring 3 cups broth and all dried mushroom to oil in a large heavy saucepan. Reduce heat to medium and simmer until mushrooms are soft, about 15 minutes. Using slotted spoon, transfer mushrooms to work surface. Cut large mushrooms in half. Reserve broth and mushrooms separately.
Melt 1 tablespoon butter with 1 tablespoon olive oil in heavy large skillet over medium-high heat. Add fresh mushrooms and saute until beginning to brown, 7-8 minutes. Add reserved soaked mushrooms and saute 5 minutes longer. Remove from heat.
Melt remaining 2 tablespoons butter with 1 tablespoon oil in heavy large saucepan over medium heat. Add shallots and garlic; saute until shallots are soft, about 3 minutes. Add farro; stir 1 minute. Pour in reserved mushroom soaking broth, leaving any sediment behind. Bring to boil; reduce heat to medium, cover, and simmer until almost all liquid is absorbed, about 10 minutes. Add 3 1/2 cups chicken broth. Boil uncovered until farro is tender, adding more broth by 1/2 cupfuls if dry and stirring occasionally, about 18 minutes longer. Stir in cheese, cream, and herbs. Stir in half of mushroom mixture. Season with salt and pepper. Transfer to bowl; scatter remaining mushrooms over top.