I've seen on Extreme Couponing where the people would have tons and tons of inserts. I know certain elements of the show are fake, just like numerous shows but it's funny when they say people gave them the inserts. I don't even know enough people who get the Sunday paper to even have 10 inserts for that matter. It is a Neat show but they stretch a lot of what of what's going on in my opinion.
You can buy them online, but they're pricey. A lot of people go "dumpster diving" and get them from recycling centers. If you have a friend or family member who delivers newspapers, you can also get them for free. I use coupon clipping services where I can order multiple individual coupons. They're cheaper that way.