Welcome to MobileRead Forums.
Have you tried
http://www.readwithoutpaper.com/ ? I've been able to buy some books from there which were restricted from other sites. Borders also has ebooks which are restricted on other sites - though I agree it's a hard site to search through.
And there are lots of ebook stores which cater for less well known authors, and which usually don't restrict or use DRM. Try googling for BeWrite, Double Dragon, and Direct ebooks for example.
Regards, Alex
PS. The classic Sci Fi non restricted non DRM site is Baen's at
http://www.webscription.net/