I found a couple of interesting cases for the 505, and apparently for the Kindle, too. The cases are very padded with what seems to be a memory foam and hold the reader in with the corner straps. They zip closed. They are at amazon.com for just under $22 each.
One has a really nice antique map design, and it has an external pouch to hold cables, earbuds, what-have-you.
http://www.amazon.com/CaseCrown-Doub...1441494&sr=8-4
The other is is a black nylon case with no external pocker.
http://www.amazon.com/PRS-505-Digita...1441744&sr=8-2
Edited to add: These cases are sold by Senyx at amazon. There is also a Senyx store at eBay.
http://stores.shop.ebay.com/senyx-di...34Q2ec0Q2em322