Purchase truffle oil at gourmet shops or the gourmet section of well-stocked grocery stores.
There is no way to just look at a truffle oil and predict the quality. Purchase from a reliable source and price will be the best indication of quality.
Spray 8x8x2-inch baking dish with nonstick spray. Heat 2 tablespoons olive oil in heavy large skillet over medium-high heat. Add garlic; stir until beginning to brown, about 30 seconds. Add all mushrooms; sauté until soft, about 5 minutes.
Add chicken broth; bring to boil. Gradually whisk in polenta. Reduce heat to medium and cook until polenta is tender and mixture is very thick, stirring frequently, about 10 minutes.
Stir in truffle oil. Season to taste with salt and pepper.
Transfer to prepared pan, spreading evenly. Press plastic wrap onto surface of polenta.