Sorry, the government $40 coupon program has been over for several years. The "coupons" were actually like pre-paid debit cards that expired 90 days after issue. If you should happen to have one, or if you find one of these coupons for sale on Craigslist or Ebay, they have expired and they are worthless.
That being said, you still have a number of options:
Try your local Thrift Store: people often give them to charity when they no longer need converter boxes. Garage sales are also a good source, as people upgrade their old TVs. Do make sure to CHECK them out and make sure they work first! Sometimes they will be missing their remote control unit, but you may be able to buy a replacement for about $10.
Walmart, Best Buy and other Discount Stores usually carry them, or you can try Ebay. You can try searching Amazon. Another option is to replace your current TV with a new TV, which will both get you free broadcast TV and give it to you in High Definition. Smaller models, 15 to 30 inches, are quite reasonably priced now.
If you are near the Canadian or Mexican border and currently receive analog signals from foreign stations, or if you have been receiving a LP analog translator, the Converter Box needs to have a function called "analog pass thru" for it to be able to receive those stations. Many Canadian and Mexican stations are now converting to digital or they will in the near future. If You need a box with pass thru, but can't find one, you could use an "a-b" switch to switch from digital to analog.
Make sure you have a proper antenna for it, rabbit ears usually don't work very well. We can help you select the right antenna for your location. If you have any more questions, please ask.
Looks like the converter boxes listed on Amazon are coming with a lot more features than they did when the government first rolled out the program to get us off of analog.
On a quick search, for about $40 USD, you can get the HomeWork HW-150PR model that includes PVR capability.
For $60, you can get the iView HDTV 3500 STB Converter Box bunder that includes an indoor antenna, HDMI cable, and more importantly a remote control for the lazy ones of us