Yeah, you can purchase them. One common one is purchasing settlements (lawsuits, injuries, etc.) that are paid out over time.
Discounted notes exist for all kinds of things, you'd be surprised.
Finding them is the trick. There's no market where you just go purchase them. The people who do this have a business doing so.
They advertise, and send yellow letters.
It's not worth it on a small scale, IMO, only if you want to start an ad business, basically. The amount of volume you have to send to get hits, and the amount of work, makes it infeasible to do one-offs.