Check out Spiral Staircase v1 Architectural Stairs 360 3D model on CGTrader for $22. High-quality mesh by modernarchstudio.
STL (Stereolithography, filesize: 63.1 MB), OBJ (OBJ, filesize: 166 MB), 3DS (3D Studio, filesize: 32 MB), DWG (AutoCAD, filesize: 32.9 MB), 3DM (Rhinoceros 3D, filesize: 10.8 MB)