Given that the Empire has a whole galaxy to plunder for raw materials, I don't think it'd be that hard. Expensive as all hell, due to the logistics, but not technically hard. All you'd need to do is find a moon or planetoid made of iron, and build a big steelworks on it.
Also, hasn't Star Wars got weird technology that means you can make metal transparent (I think I read somewhere that windows were made of it on capital ships, though I could be making it up) and the like? So In theory, you could just get a planet, take it to bits and convert the matter into whatever material you needed to build the ships.
I dunno, I know nothing about Star Wars to be honest, but given the vast scale of the Republic/Empire, I think it'd be quite feasible to build something that big. Whether it'd be practical is another question. ("Yes, we can build you a great big ball covered in guns. Just come back in 300 years!")