They're grouped in "miscellaneous," and there aren't many of them. Open the Miscellaneous Ebooks forum (
https://www.mobileread.com/forums/ebooks.php?forumid=138) and use the "search ebooks" button to look for "ereader" or "pdb." (You get more hits with ereader, but I think there are a few listed as pdb and not ereader.)