I realise this is an old thread but……..
Alcoa (Architectural aluminium) stock/distribute a modular staircase in various shapes n sizes.
For the more discerning, Spiral Staircase Systems in the UK have been in the business for nearly 40 years, and have put staircases in arab palaces, HongKong banks, and Collins’s baracks museum, tho’ they are not particularly proud of that one..the architect insisted on unpainted steel…which promptly rusted, before they got the environment controlled.