You don’t absolutely need metal weights to enhance your strength and enlarge your muscles. You can just do pull-ups and use your entire body weight instead. Even if that’s not enough weight, you can always wear a weighted vest to make things more challenging.
But it’s also a challenge to find the best place tom do your pull-ups. Your best solution is to just get a pull-up bar which you can place over your doorway. The best pull-up bar should be able to support your weight (or even more) and it should still keep your door frame from damage. Of course, it should fit into your door frame (the best ones are adjustable, and it should allow for multiple grips.
You should pay attention to how these things are installed. Some use brackets and screws to make them more secure, though others can easily be detached so you can use the device for pushups and sit-ups as well.
Here are some of the best options you have for pull-up bars: