I think you need a Pokemon with a specialy natural ability that will dig up items if you walk over them.
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In the underground
Use the itemfinder at the spot where mr fuji once stood at the top of Pokemon tower you should dig up a soothe bell. where mr fuji was standing use your itemfinder there
well, there are tons of items, the main items are spheres (those let you buy things), fossils (you can go to oreburg and turn those fossils into other pokemon), and many many more.
You can't grow berries in Leafgreen, so get 6 meowth. (Pickup ability) battle low-level Pokemon, checking the party every now and then. If they don't have items already, Meowth have a chance of obtaining a berry for you after each battle.
Items cannot be buried, you simply cannot see them with the pixel eye. You need to use a special machine called a Dowsing Machine (or sumin like tht).