The golden well (Thueringen Lakeside, E6; Channel 1 ONLY) can be used to harvest Bellem's Boxes. Simply drag gold pieces from the inventory and release on the well. You will then be prompted to input the amount of gold pieces to be dropped. Dropping gold pieces into the well gives you a chance of having a Bellem's Box going directly into your inventory.
Most people I know drop about 40-50 pieces at a time, resulting in about 50% chance of getting a Bellem's Box per drop. Furthermore, there is a chance of getting jackpot, which is when the well returns pure gold bars and gold pieces. The gold bars are sometimes enough to cover the cost of gold pieces, so the Bellem's Boxes can be sold for profit. (In Carracci, a Bellem's Box can be sold for 17,000-20,000 vis.)
Using a Bellem's Box will result in a random item being dropped on the ground. It is usually a healing potion or a spinelle, but sometimes may be a weapon, armor, or costume. Most people seem to open the boxes in large quantities (100+) at one go.
Gold pieces are dropped by monsters in Dr. Torsche's Mansion and El Tejado Verdas. They can also be bought from the precious metal merchant in Auch (G7) for 165 vis each.