Depends where (or if) you work.
I know some pikers who will just go pull cardboard out of Dumpsters, and then complain when their house is crawling with Silverfish.
I know another guy that works in a very large printing factory, and is able to bring home bubble wrap and cardboard all day long.
Some people are in situations where they have easy access to quality cardboard.
If you look around at work, regardless of your job, you might be surprised at what kind of useful packing supplies you find. Or if you're on good terms with someone that works in certain retail environments where they have so much cardboard that they consider it a nuisance. Before I was buying skids of custom cut cardboard from a local box manufacturer, I used to go over to the 24 Hour pharmacy I live next to and ask them. They don't have much to do at night other than stock the shelves, so they were always happy to grab some boxes for me. Plus I would always make a point to be nice to everyone when I was in the store.
Cardboard is cheap and easy. I find the bubble wrap to be more expensive, and harder to free-source.