Soulbound Weapons (SBW) can be obtained in various ways. SBWs enhance a Hero's Skills, and each Hero has their own SBW.
For more information regarding a Hero's SBW and the added Skill effects you can go to Facility > Library of Dimensions > Hero Tome > Select any 4-Star and above Hero.
You can forge Old Weapon into a random SBW.
When forging you are given 2 Weapons that you can choose between. You also have the option to re-roll the 2 Weapons.
Golden Weapon Box
When opening a Golden Weapon Box, there is a chance of receiving a 4 to 6-Star SBW. You also have the chance to receive Old Weapons, to forge more SBWs.