I need a substitute for a punching bag, if there is such a thing.

You can make your own boxing bag easily, all you need is the right kind of bag. I use an old army canvas bag for mine. Fill a small layer of sand at the bottom, this will give it weight and make it more stable. The inner piece, there are two ways to do it. The first is to just fill it up with scrunched up paper balls. The other is to take 2l plastic bottles and fill them with water, and use them in the center, surrounded by paper balls. Warning though. The plastic bottles will make the bag MUCH harder. This is a good thing later on when you're good at punching, but you'll regret it as a beginner trainee. Also, be very carefull about seams on the bag, they are usually what end up hurting your hands. I'd definately wrap your fists if you're planning a long session.