If you cannot find a converter box in stores then you need to make your purchase online. Physical Retail stores have not been able to keep uo with customer demand for converter boxes but there are a lot of online retailers that have plenty of stock. Most notably is Solid Signal. They have over 25 different converter boxes in stock right now and orders are suppose to be received within 4 days.
There are also many other online retailers that have a lot of converter boxes. It would be better to buy online rather than let your coupon expire.