Speaking as someone who owns an ereader with a small market presence, you might want to try my approach. I like the slip-case that I got with my ereader, but it's not really a cover and provides very little protection while traveling. After poking around the various discount stores, etc., I discovered a zip-up case for a Nintendo DS that fits my JetBook Lite perfectly. It zips closed, holds the ereader with elastic straps that don't interfere with reading, and stays open when I unzip it. I don't use it often, just when traveling, but it does a great job.
Have you considered non-traditional sources?