It can be found in various locations around Skyrim, or randomly obtained from defeating the Keepers in Soul Cairn. It can be crafted at a forge. It is also a possible weapon of a summoned Wrathman, though it is not lootable.
The Dragonbone Greatsword requires a Smithing level of 100 and the Dragon Armor perk to create. It can be upgraded with a Dragon Bone at a grindstone and also benefits from the Dragon Armor perk, which doubles the improvement. [Source: wikia]
You can download the papercraft model template here: The Elder Scrolls V: Dawnguard - Dragonbone Greatsword Free Papercraft Download