During the quest "Hide and Seek," Gunmar tells the Dragonborn to go kill a vampire, who is usually found in an inn. The vampire is not hostile and wears vampire robes and vampire boots. Kill the vampire and take the boots.
They can be difficult to get in Dawnguard as only certain vampires wear them. As a vampire, however, there are two spawns on either side of the table in the room with coffins in Castle Volkihar.