Raise pigs on your farm
Build a Deluxe Barn
Keep pigs outdoors during non-winter seasons
Let pigs roam outside during the day
Wait for pigs to dig up truffles on the ground
Pick up the truffles before the day ends
Use the Foraging skill to increase truffle value
Process truffles into truffle oil with an Oil Maker
Buy truffles from the Traveling Cart when available
Find truffles in the Desert Festival shop when offered