Current and future space vehicle resources?

Hey, I realize that this isn�t the problem now but in the nearing future resources will become more and more scarce... I want to know how we would/will be able to continue space exploration if the special metals and titanium supplies are nearly depleted. What then? Also, in the future more likely when than if we build giant 250+ person space ships, how long might these take to build? Will they be build in orbit of this planet, another planet, or the surface of a planet? Again, where will we get the metals? By mining asteroids? That might take years to get the metals required to build just a replica of the current space shuttle....