There are currently two different ways to obtain a Greenstone Gold Sledge Hammer. If you are playing in Creative Mode, you may acquire them at the Item Shop.
The other way, which is the only way to obtain them in Dig Deep mode, is to mine all the way down and prospect bedrock (Press 'Y' in order to prospect it). Once you have prospected bedrock, you will see a message notifying you that "Spiders will now drop Sledge Hammers". Once you receive that message, Spiders will now be able to drop the Sledge Hammer, however it may take several hours, even with a mob/spider Farm, to actually acquire this item.