Spiral Staircase v3 Architectural Stairs 360 3D model is available on CGTrader for $22. Original author: modernarchstudio.
STL (Stereolithography, filesize: 827 MB), OBJ (OBJ, filesize: 396 MB), 3DS (3D Studio, filesize: 74.1 MB), DWG (AutoCAD, filesize: 39.7 MB), 3DM (Rhinoceros 3D, filesize: 11.9 MB)