The time has come and gone, I'm afraid. You can sometimes find them on eBay or Amazon or from other places, but the program has gone bye bye. You are probably better served by buying a new TV that doesn't need one.
All 'free' coupons expired many years ago. You have two real choices: buy a new converter box or buy a new TV. Although used converter boxes are available on Craigslist, at Thrift Shops and at garage sales, many are non-functional which is why they are for sale.
You can get a Mediasonic converter - new - for about $30. I've seen them and several similar units for sale at Walmart and other stores. They also function as a PVR if you plug in a USB drive.
Here it is on Amazon: http://amzn.to/2e34Cqd