Ok, yes of course, I know what type of round glasses you are talking about, and as a matter of fact, what letterman wears in public places is called metal round glasses, which have been very popular around the world. And currently, there are a lot of places where you could get them, just try to take a look at Amazon or Alibaba, where there would be more options and better prices.
The round glasses that david letterman wears are so creative and fashionable. The round glasses are very suitable for those people with long shape face and pointed chin. If you like to buy this kind of glasses, you could go to the amazon or ebay to find the similar type of the round glasses. You could find that the price in the online store is relatively cheap.
Well, it's so cool and fashionable when David Letterman wears the round glasses. I made a deep search online and found that David Letterman is a fan of ray-ban glasses. These round glasses can be found on many sites, such as ebay, alibaba and walmart. But here I'd like to recommend you that walmart may be your best choice. There are thousands of eyeglasses on walmart.com and at walmart stores. My friends are fans of Brad Pitt and Justin Bibber, even PSY and they found these famous people stylish eyeglasses on walmart.