Here is a case from casecrown for 5.21 for the kindle 3. You have to dig on amazon as its not listed in the kindle store, actually in electronics. I bought a similar case for the kindle 4 NT for the same price, from the same seller. For 5.21, you can't beat it and it will do a much better job protecting the screen than a sock. Comes in a variety of colors. I opted for black.
http://www.amazon.com/CaseCrown-Keyb...uct/B0042I5I6C