Check with CivAssistII and look at the resource locator on the Miscellaneous tab. See if there are other sources that are just hiding from you. If not, figure out why the other civ won't trade. Do they not know the aluminum is there? Teach them rocketry. Are the resources not connected? Send over some workers to connect it. Are they trading it to someone else? Drag them into a war to break the trade deal. Can you just not afford it? Put your tax slider to 100%, change scientists to taxmen, maximize commerce by choosing tiles to work and hiring taxmen, sell whatever you can to other civs. If your aluminum bill isn't something you can afford to pay, start your spaceship parts and then break the deal. Obviously, don't do this until you have all the required techs, so you can start all the parts. Prebuild using something else in the mean time.