I wouldn't exactly consider copying from Valve's maps stealing, since nobody is even going to care. If it came from the original game, the content is already there, so it's not like some mapper is being ripped off. In any case, You still gain a lot of experience from rebuilding maps like that, so you should definitely take that route.
If you're really intent on seeing some of the source files, you could download the full SDK, which includes c1a0, and a small map containing Gordon's locker.