I wanted to point out a few other sleeves that caught my eye. I just saw this one on Amazon for the first time. No reviews yet, but the design is very clever - it looks just like run-of-the-mill manila envelope, but it comes in genuine leather and synthetic leather versions and is less than $20. I also saw a version in black synthetic leather.
manila genuine leather version
manila synthetic leather version
black synthetic leather version
I liked this one in their "Quorky" style as well (I think they use the same Kindle Keyboard product photo on all of their items in this style):
Quorky case
Finally, I saw this neoprene sleeve at my local Tuesday Morning store for about $15, in the Vine and Scatter Dot patterns:
Built neoprene sleeve