I'm not sure if ebay urls will work, but this guy sells a killer sleeve, and it looks like he lowered his price. I bought a couple for me and some of my friends and family. They're very durable, and look great.
Here is a better picture of the same sleeve, sold by another user. It's black and it has an apple logo on the front. http://i4.ebayimg.com/03/i/04/ag/a6/66_1_b.JPG
If these don't work, just do an ebay search for "Powerbook 12 sleeve" and it's the Buy It Now for 12.98.