Garelick Universal Boat Seat Swivel- Removable Quick Release in the RV Accessories department at Lowes.com
2 PCS Boat Seat Swivel Coated Mount Base Heavy Duty DIY Chair Barstool Base | eBay
Amazon.com : SeaLux Universal Heavy Duty 360 degree Seat Swivel Base 6" x 6" Mount Plate for Bar Stool, Chair, boat or van pilot seat : Sports & Outdoors