I got a great sleeve from Berties Closet
on Etsy.com, a marketplace for handcrafted goods. In fact, there are a whole bunch of handcrafters on Etsy who make ereader sleeves and covers (better for sleeves than covers, though). Just do a search for "ereader sleeve" or "ereader case."
The nice thing about Etsy is that since everything is handmade, you can request custom sizing and features. The sleeve I got was posted as a Kindle sleeve, so I asked the seller to adjust the sizing to fit my 350. It fits perfectly, is padded with foam, and is extremely well-made.
Sellers are from all over the world and many will ship internationally.