How can I get a truffle worm?

If you don't already have a farming location set up, then really the only way to find them is to be patient.

If you want to set up a farming area for them, then head on down to the cavern layer (that's the layer where the natural background wall is grey instead of brown). How deep you need to go depends on your world size, and the depth varies between worlds.

Once you're at the right level, you're going to want to clear out a pretty large area (about 1 screen high and 2 screens wide as a ballpark figure - having a smaller screen or lower resolution here is an advantage).

Once you've got your area cleared build a large box out of mud, with horizontal bars of mud every so often (I'd suggest a 4 tile gap so you can move along each level with the flying saucer). I'd suggest leaving an empty column down the middle of the box, but that's up to you.

Now that your basic structure is in place, plant some Mushroom Grass Seeds in the mud (or use the Clentaminator to convert Jungle Grass to Mushroom Grass) and wait for it to grow. You'll also need to take precautions against the Hallow and the Corruption/Crimson. The active biome doesn't actually matter, but you don't want your mushroom grass being converted!

According to the Wiki, Truffle Worms are classed as enemies, which means that you can raise their spawn rate through the use of Water Candles and Battle Potions. Pop a few Water Candles down in the area and repeatedly move from one side of the box to the other. A Hunter Potion will help you spot the worms more easily.

Note that having NPCs based nearby will reduce the spawn rate of Truffle Worms.