The schematic can be purchased for 190 caps from a random vendor - try the Watoga Market vendor in Cranberry Bog. To get the Artillery Cannon, you first need to obtain the Artillery schematic. Another set of Level 15 Raider armor can be found on the high bridge in the first region. There will always be a Power Armor Frame and Power Core near the rollercoaster ride. You can also get one in Camden Park, in the southwest area of the map. You can get a Power Armor Frame in the Morgantown Trainyard, straight east from Vault 76. The armor has a bonus stat that adds +100 to carrying weight. The HQ is located in the southwest corner of the southern mining region. The quest can be started by traveling to the Garrahan Mining Headquarters and examining a poster. Successfully complete the 'Miner Miracles' quest to unlock the Excavator armor schematic.