Kottke has an interesting article on newspapers used in TV shows, due to clearance time and costs they use fake ones supplied specifically for this purpose. This is an example of one of those things you don't notice on screen yet requires a fair bit of time, effort and thought. There are hundreds of these sorts of things on every set that most people will never even be aware of.
It's also another example of a business that supplies something you just never think about. If you think of all the things that surround our lives you can probably think of a few businesses that supply odd things you often overlook. Companies that do nothing but supply screws, or electrical sockets, but there are loads. Think about companies that produce ballcocks for toilets, who manufacture different types of putty, who churn out various food containers or make rubber seals for car doors.
Even simple things. Take a mug, yes there's a company that supplied the mug to the store, but you'll probably find that it was supplied the mug by a company that churns out blank mugs in a variety of shapes, who in turn buys the clay/porcelain/whatever from a company that does nothing but produce that. Have a wander around any industrial estate and you'll find companies supplying things you'd never think of but have to exist.