Windows 8 RTM is available for all MSDN and TechNet subscribers. Oh, and Visual Studio 2012 is available for everyone!
How can you get them?
Just in case your customer doesn't have MSDN/TechNet subscription - here's *free* Windows 8 Enterprise 90-day evaluation for all developers (link: http://j.mp/NHWZzz )
Happy Eid, Happy coding!